1. Make the gratin topping: In a small mixing bowl, combine panko, butter, parsley, 1 1⁄2 tsp. lemon zest, 1 oz. gruyère, and salt and pepper to taste. Toss gently and set aside.

2. Heat a sauté pan over medium-high heat. Add the shallots and bacon, stirring frequently until the bacon is rendered and the shallots begin to caramelize, about 7-9 minutes. Add Brussels sprouts and garlic and cook until they are just beginning to brown, about 6 minutes. Deglaze the pan with the lemon juice and remove from heat.

3. Make the Mornay sauce: In a 2-quart saucepan, melt the butter. Add the flour to make a roux and cook until it smells toasted and the color is a light brown, about 30 seconds. Add thyme and cream, stirring vigorously with a whisk to avoid lumps. Heat to a simmer, stirring continuously until the sauce thickens, about 3-5 minutes. Remove from heat and pour the sauce through a strainer. Return the sauce to the saucepan and, over very low heat, add remaining gruyère and parmesan, stirring until the cheese has melted. Season with 1⁄2 tsp. salt and 1⁄4 tsp. pepper.

4. Pile the sprouts into a 10-inch oval gratin dish. Pour the Mornay sauce over the sprouts and sprinkle the breadcrumb mixture over the top. Cover loosely with a sheet of tin foil and bake until bubbling, about 15 minutes. Remove foil and cook another 12-15 minutes until the top is nicely browned. Remove from oven and let rest for 10 minutes. Serve warm.