Portabella Mushroom And Vidalia Onion Gratin

Portabella Mushroom And Vidalia Onion Gratin

Servings 8 -10
Author Melissa Sperka


  • 1 pound baby portabella mushrooms sliced
  • 1 x-large vidalia onion sliced into small wedges [or 2 medium]
  • olive oil
  • salt & black pepper
  • 3 cloves garlic minced
  • 2 tsp fresh thyme
  • 3 Tbsp butter divided
  • 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
  • For the crumb topping:
  • 1 Tbsp butter melted
  • 1 1/2 cups fresh bread crumbs
  • 1 Tbsp chopped Italian parsley


  • Preheat the oven to 350°F. Butter the bottom and sides of a 1 1/2 quart baking dish, then set aside.
  • In a heavy saucepan, saute the mushrooms and sliced onions in a couple of drizzles of olive olive until the mushrooms are golden and they've released their liquid. Season with salt and pepper to taste. This step will take around 12 minutes.
  • To the mushrooms, add the minced garlic, fresh thyme, 3 tablespoons of butter, 1 tablespoon of red wine vinegar and 3 chopped green onions.
  • Continue to saute over medium heat for 2-3 minutes until the garlic has cooked and the butter has melted.
  • Add 2 1/2 tablespoons of flour and cook for 1 minute 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 muenster cheese and 1 tablespoon of chopped Italian parsley to the creamy mushrooms. Cook just until the cheese melts.
  • To make the crumb topping: In a small bowl melt 1 tablespoon of butter in the microwave. Toss the breadcrumbs, chopped Italian parsley and garlic salt with the melted butter to make the crumb topping.
  • Pour the mushrooms into the buttered baking dish and sprinkle the remaining shredded muenster cheese on top.
  • Spread the buttered crumbs evenly over the cheese.
  • Place into the oven and 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 it's saltiness. Gruyere cheese also works beautifully in this dish!