Cheesy Brussels Sprouts Gratin

Name: Cheesy Brussels Sprouts Gratin
Ingredients: 8 ounces pancetta, cut into ½-inch pieces; 2 pounds Brussels sprouts, trimmed, halved, quartered if large; 2 shallots, thinly sliced; 3 garlic cloves, finely grated; 4 tablespoons melted unsalted butter; ½ teaspoon freshly ground black pepper; 2 tablespoons thyme leaves, divided; 2 cups heavy cream; 1 tablespoon Dijon mustard; 1 cup walnuts; 2 ounces Gruyère, grated (about ⅔ cup); 2 ounces Parmesan, crumbled (about 1 cup); 1 teaspoon crushed red pepper flakes