Gratin of Mushrooms and Onions
Serves 8 as appetizer, 4 as main course over salad
4 moderately large yellow onions (2 pounds)
1 pound crimini mushrooms
½ pound shiitake mushrooms
3 tablespoons butter (plus more for the dish)
1 tablespoon olive oil
½ teaspoon ground allspice
1 teaspoon each fresh thyme and rosemary, chopped
Salt and freshly-ground black pepper
¼ cup heavy cream
½ cup grated Parmesan cheese
1/3 cup grated Gruyere cheese
1 baguette, sliced a half-inch thick, slices toasted if you’d like
Peel the onions, cut them in half through the poles, then slice each half crosswise into thin slices. Heat 2 tablespoons of the butter in a large nonstick skillet with a lid over medium heat, add the onions along with some salt and pepper, and the allspice, thyme, and rosemary. Stir to coat. When everything sizzles, cover the pan, turn down the heat to low, and let the onions cook for 15 minutes. Remove the lid and raise the heat to medium. Continue to cook the onions until they just start to brown, stirring occasionally. Spoon the onions into a bowl and set aside.
While the onions are cooking, prep the mushrooms: wipe them all with a barely damp paper towel to clean them. Cut off the end of the stem of the criminis, and slice them fairly thick. Remove the shiitake stems and slice the caps the same way. Once the onions are cooked, heat the remaining butter with the olive oil in the skillet. When it stops foaming, watch until the mixture heats a bit more, just barely shimmering. Add the mushrooms and spread them out quickly. Raise the heat to medium-high and cook the mushrooms, without stirring, until they brown on the bottom. This will take a minute or two. Put on the lid and lower the heat to medium. Cook for two minutes, the remove the lid. Put the onions back in the pan, mix everything up, then turn up the heat. Cook the mixture, stirring, until the liquid is almost evaporated. Taste for salt and pepper. You don’t want too much salt, since the cheese is also salty.
While the mushrooms are cooking, place an oven rack about six inches under the broiler, and preheat the broiler. Butter a shallow baking dish (a shallow 13 x 9 – inch dish works well), then spoon in the cooked onion/mushroom mixture. In a small bowl, beat the egg yolks and cream with a pinch of ground pepper. Beat in the grated cheeses. Spread the topping over the onions and mushrooms, then put it all under the broiler for a minute or so, until it’s nice and golden. Let it sit for five minutes, then serve on the baguette slices.