For the topping:
3 cups coarse fresh bread crumbs
3 Tbs. unsalted butter, melted
1 1/2 tsp. lemon zest
1/2 tsp. kosher salt
1/2 tsp. freshly ground pepper
2 Tbs. minced fresh flat-leaf parsley
For the gratin:
1 Tbs. unsalted butter
Kosher salt, to taste, plus 2 tsp.
2 lb. brussels sprouts, trimmed and halved lengthwise
2 Tbs. all-purpose flour
8 oz. Gruyère cheese, grated
1/2 oz. Parmigiano-Reggiano cheese, grated
1 tsp. lemon zest
3/4 tsp. freshly ground pepper
1 cup heavy cream
1 jar (8 oz.) caramelized shallots, liquid strained off
Preheat an oven to 375°F.
To make the topping, in a bowl, stir together the bread crumbs, butter, lemon zest, salt, pepper and parsley. Set aside.
To make the gratin, grease a 12-inch ovenproof nonstick fry pan with the butter. Bring a large pot of salted water to a boil over high heat. Add the brussels sprouts and cook until just tender, 5 to 7 minutes. Drain, then transfer to a bowl of ice water. Drain and pat dry with paper towels.
In a bowl, stir together the flour, Gruyère, Parmigiano-Reggiano, lemon zest, the 2 tsp. salt and the pepper. Stir in the cream, then the brussels sprouts and caramelized shallots. Transfer the mixture to the prepared fry pan and smooth the top. Sprinkle the bread crumb mixture evenly on top. Bake until the bread crumbs are golden brown, about 35 minutes. Let the gratin rest for 15 minutes before serving. Serves 8.