Crunchy Mustard Chicken Bake

Name: Crunchy Mustard Chicken Bake
Ingredients: 8 chicken thighs, bone-in, skin-on (3 1/2 to 4 pounds); 4 large cloves garlic, thinly sliced; 3 large shallots, thinly sliced; 3/4 cup dry vermouth; Extra-virgin olive oil; 1/2 cup Dijon mustard; 1/4 cup lemon juice (about 2 large lemons); 2 tablespoons fresh thyme; 2 teaspoons crushed red pepper; Kosher salt; 3 small leeks, root and dark green top removed; 1 1/2 pounds Peewee Dutch Yellow potatoes (see Cook's Note); 1 medium red onion, thinly sliced; 2 tablespoons unsalted butter, plus 3 tablespoons melted butter; 1 tablespoon all-purpose flour; 1/2 cup chicken stock; 1 1/2 cups fresh breadcrumbs, preferably from sourdough bread (see Cook's Note); 3 tablespoons chopped fresh tarragon; 1 teaspoon crushed sea salt flakes; 1/2 teaspoon freshly ground black pepper