8 tablespoons (1 stick) unsalted butter, melted and cooled
For the Vegetable Mixture
2 tablespoons olive oil
heaping 1/2 cup diced yellow onion
1 clove garlic, sliced
heaping 2 cups trimmed and quartered Brussels sprouts
3 medium carrots, peeled and sliced into 1/4 inch slices
heaping 2 cups trimmed and quartered cremini mushrooms
1 teaspoon roughly chopped fresh thyme
salt and pepper to taste
2 tablespoons all-purpose flour
1 cup chicken or vegetable broth
1/3 cup heavy cream
1/2 cup finely shredded gruyere cheese
Place a rack in the center of the oven and preheat oven to 350 degrees F. Lightly butter a 9-inch pie dish (a deep one), and set aside.
In a medium bowl combine crackers, bread crumbs, cheese, pepper, thyme, and butter. Toss to coat everything in butter. Spoon just less than half of the mixture into the bottom of the pie dish and slightly up the sides. Place in the oven and bake for 12 minutes, until just golden brown. Remove from the oven and set aside to cool while you make the filling.
Heat olive oil in a large skillet over medium heat. Add the onions and saute until softened and browned, about 5 minutes. Add the garlic and saute for 1 minute more. Add the brussels sprouts, carrots, mushrooms and thyme. Increase heat to medium-high and let the vegetables saute down until well softened, browned, and caramelized, about 15 to 20 minutes, stirring occasionally. Add salt and pepper as the vegetables cook down.
The mushrooms will release a lot of their moisture and then the mixture will brown and cook down.
When vegetables are cooked down and softened, reduce heat to medium, sprinkle the flour over the vegetables and stir until the flour disappears. Slowly add the chicken stock and stir, scraping any brown bits off the bottom of the pan. Bring to a simmer to thicken. Finally, stir in the cream and remove from the heat.
Taste and season with salt and pepper if needed.
Spoon mixture onto the browned crust. Sprinkle with gruyere. Top with the remaining crumble crust. Place in the oven to bake for 12 to 15 minutes, until top crust is golden brown. Remove from the oven and allow to cool for 15 minutes before serving warm.