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Baked Portabella Mushroom And Vidalia Onion Gratin

Portabella Mushroom and Vidalia Onion Gratin

Course Side Dish
Cuisine American
Keyword mushroom-onion-gratin, portabella-mushrooms, vidalia-onion-recipes
Prep Time 20 minutes
Cook Time 30 minutes
Resting Time 5 minutes
Total Time 50 minutes
Servings 8 servings
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.