Mushroom Popover Casserole
- *** MUSHROOM FILLING:
- 4 1/2 tbsps olive oil
- 1 lb brown mushrooms (cremini), thinly sliced through the stems
- 1 cup chopped green onions (including 2 inches of the green stems)
- 2 tbsps finely chopped garlic
- 2 tsps crushed dried rosemary
- 2 tsps dried thyme
- 1/2 tsp kosher salt
- freshly ground black pepper
- 1/2 cup mascarpone cheese
- *** POPOVER:
- 4 tbsps unsalted butter
- 1 1/2 cups all-purpose flour
- 1 tsp kosher salt
- 1 1/2 cups whole milk
- 4 large eggs
- grated Parmesan cheese, preferably Parmigiano-Reggiano, for sprinkling
Arrange a rack in the middle of the oven and preheat to 450°F/230°C. Have ready a 9-by-13-in/23-by-33-cm baking dish.
FOR THE MUSHROOM FILLING:
Heat the olive oil in a large, heavy frying pan over medium-high heat until very hot but not smoking. Add the mushrooms and cook, stirring often, until any liquid exuded from the mushrooms has evaporated, 5 to 6 minutes. Add the green onions and garlic and cook, stirring, until the green onions have softened, about 2 minutes. Stir in the rosemary, thyme, ½ tsp salt, and several grinds of pepper.
Remove the frying pan from the heat and add the mascarpone, stirring until it has melted. Season the mushrooms with more salt and pepper if needed. (The filling can be prepared 1 day ahead; cool, cover, and refrigerate. Reheat over low heat, stirring.)
FOR THE POPOVER:
Put the butter in the baking dish, and put the dish in the oven until the butter has melted and is hot, 5 minutes.
Meanwhile, in a medium bowl, mix together the flour and salt. In a large bowl, whisk together the milk and eggs. Add the flour to the egg mixture, whisking until no lumps remain and the batter is smooth.
Remove the hot baking dish from the oven and, using pot-holders, tilt the dish several times to spread the butter evenly over the bottom. Pour the popover batter into the dish; bake 15 minutes. The batter will start to puff up around the edges. Reduce the temperature to 350°F/180°C, and bake for 15 minutes more. At this point, the sides will have puffed up around the edges of the baking dish.
Remove the dish from the oven (close the oven door to maintain the temperature). Quickly spoon the mushroom filling over the bottom of the popover. Return to the oven and bake until the mushrooms are very hot, 10 to 12 minutes more.
Remove the popover from the oven. With a sharp knife, cut into six to eight portions. Serve hot and pass a bowl of Parmesan cheese for sprinkling.