- 5 tbsp salted butter divided
- ¼ cup chopped yellow onion
- ¼ cup chopped green bell pepper
- ¼ cup chopped celery
- 2 scallions thinly sliced
- 1 tsp minced garlic
- 2 16 oz containers fresh oysters, drained well
- 4 oz fresh mushrooms, sliced about 1 1/2 cups
- 2 tbsp all-purpose flour
- ½ cup heavy cream
- 1 oz Parmesan cheese, grated about 1/4 cup
- ¾ tsp kosher salt
- ¼ tsp black pepper
- ¼ tsp ground nutmeg
- 1 cup coarse fresh breadcrumbs from 1/4 baguette
- Preheat broiler. Melt 2 tablespoons of the butter in a skillet over medium-high. Add onion, bell pepper, celery, scallions, and garlic. Cook, stirring, until vegetables soften, 5 to 7 minutes. Add oysters and mushrooms. Bring to a simmer, and cook, stirring often, 5 minutes. Set aside.
- Melt 1 tablespoon of the butter in a small saucepan over medium. Whisk in flour, and cook, whisking constantly, until smooth, 30 seconds to 1 minute. Add cream, and cook, whisking constantly, until very thick and beginning to bubble. Add Parmesan, and cook, whisking constantly, until cheese is melted. Remove from heat.
- Using a fine wire-mesh strainer, strain oyster mixture; discard liquid. Add oyster mixture to cheese sauce; stir until fully incorporated. Stir in salt, pepper, and nutmeg. Spread mixture in a lightly greased (with cooking spray) 11- x 7-inch baking dish. Melt remaining 2 tablespoons of butter; toss breadcrumbs with melted butter. Sprinkle breadcrumbs over oyster mixture.
- Broil on middle rack of oven until breadcrumbs are deep golden brown and mixture is bubbly, 4 to 5 minutes.