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Portabella Mushroom and Vidalia Onion Gratin

This Portabella Mushroom and Vidalia Onion Gratin is a spectacular way to enjoy mushrooms and sweet Vidalia onions together in the form of a casserole. It’s a magnificent side dish to serve alongside steak, pork, kabobs or chicken during grilling season. Likewise, it will fit perfectly on your holiday table too, with Prime Rib or filet. It’s a special dish for a special occasion.
Portabella Mushroom And Vidalia Onion Gratin

The South’s Beloved Vidalia Onion

Vidalia onions are a sweet onion grown in Vidalia Georgia. The different varieties are unusually sweet compared to other onions due to the low amount of sulfur found in the soil in which they’re grown. So beloved are these sweet onions that they were named the official State vegetable of Georgia in 1990 and rightly so, they’re a Southern delicacy worth noting. Onions are in such abundance in the Summer months, and the flavors in this gratin are only heightened by the sweetness of the Vidalia’s.
Portabella Mushroom And Vidalia Onion Gratin

Portabella Mushrooms and Vidalia Onions Pair Beautifully in this Gratin

We love mushrooms alongside steaks or chicken either grilled or sautéed. When sautéed, I usually add minced garlic, chopped onion, a splash of balsamic vinegar, a sprig of fresh rosemary or whatever other seasonings that are ready to be used from my herb garden. This portabella mushroom and onion gratin is sprinkled with a crunchy breadcrumb topping making it an incredible way to change up your side dish menu. Filled with tasty cheesy mushrooms and Vidalia onions and a crispy crumb topping, it can be baked in one dish or you can divide it among several gratin dishes for a gorgeous presentation for individuals.

Step-by-step images to prepare Gratin

Other Ways to Enjoy Portabella Mushroom and Vidalia Onion Gratin

While this Portabella Mushroom And Vidalia Onion Gratin is primarily enjoyed on the side, it’s also delightful served as a topping for baked potatoes, ladled over egg noodles or buttered rice. Try spooning this gratin over a piece of Italian seasoned grilled chicken topped with a sprinkle of cooked pancetta or bacon. It’s heavenly! If you love Vidalia onions too, checkout my recipe beer batter onion rings, too. You may also like this recipe for Vidalia onion bake from Taste of Home.

Unbaked casserole with mushrooms and onions

Grilled Steak and Portabella Mushroom And Vidalia Onion Gratin

Baked Portabella Mushroom And Vidalia Onion Gratin

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Portabella Mushroom and Vidalia Onion Gratin

Prep Time20 minutes
Cook Time30 minutes
Resting Time5 minutes
Total Time50 minutes
Course: Side Dish
Cuisine: American
Keyword: mushroom-onion-gratin, portabella-mushrooms, vidalia-onion-recipes
Servings: 8 servings
Calories: 305kcal
Author: Melissa Sperka


  • 1 pound baby portabella mushrooms sliced
  • 1 extra large vidalia onion or 2 medium sliced into small wedges
  • olive oil
  • 1 tsp salt
  • 1/2 tsp freshly ground black pepper
  • 4 Tbsp butter divided
  • 3 cloves garlic minced
  • 2 tsp fresh thyme
  • 1 Tbsp red wine vinegar
  • 3 finely chopped green onions
  • 2 1/2 Tbsp all purpose flour
  • 1 1/2 cups whole milk
  • 2 cups shredded Muenster cheese divided
  • 2 Tbsp chopped Italian parsley divided
  • 1 1/2 cups fresh bread crumbs
  • 2 Tbsp grated Parmesan cheese


  • Preheat the oven to 350°F. Butter the bottom and sides of a 1 1/2 quart baking dish, then set aside. (Alternately use single serving gratin dishes)
  • In a heavy saucepan, saute mushrooms and sliced onions in a couple of drizzles of olive olive. Cook for 10 minutes or until mushrooms are golden. Season with salt and black pepper, adjust  to taste. 
  • To the mushrooms, add 3 Tbsp butter, minced garlic, thyme, red wine vinegar and green onions.
  • Continue to saute over medium heat for 2 minutes just until the garlic has cooked and the butter has melted.
  • Add flour and stir until the flour has coated all of the ingredients.
  • Increase the heat to medium high and add 1 1/2 cups of milk. Cook until the mixture is bubbly and has begun to thicken. Around 2-3 minutes.
  • Add 1 1/2 cups of shredded cheese and 1 tablespoon of chopped Italian parsley to the pot. Mix just until the cheese melts. Remove from the heat mixing in 1 Tbsp parsley. 
  • In a small bowl melt remaining 1 Tbsp butter in the microwave. Toss the breadcrumbs, Parmesan and remaining Italian parsley together.
  • Pour the mushrooms into the buttered baking dish and sprinkle the remaining shredded Muenster cheese and breadcrumbs on top.
  • Bake for 25-30 minutes until the topping is golden and the edges are bubbly.
  • Allow the gratin to sit on the counter for 5-10 minutes before serving to allow the sauce to thicken.


For this dish I purposely chose Muenster cheese for its saltiness. Gruyere cheese also works beautifully. Feel free to adapt using your favorite cheese in this dish.


Serving: 1serving | Calories: 305kcal | Carbohydrates: 25g | Protein: 13g | Fat: 17g | Saturated Fat: 10g | Polyunsaturated Fat: 1g | Monounsaturated Fat: 5g | Trans Fat: 1g | Cholesterol: 48mg | Sodium: 715mg | Potassium: 415mg | Fiber: 2g | Sugar: 7g | Vitamin A: 655IU | Vitamin C: 4mg | Calcium: 323mg | Iron: 2mg
Tried this recipe?Mention @melissassk or tag #melissassk!


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