Brussels Sprouts Gratin - Paula Deen

Ingredients

• 3 teaspoons Kosher salt, divided
• 3 pounds Brussels sprouts, trimmed and halved
• 2 tablespoons unsalted butter
• ¾ cup thinly sliced onion
• ¼ cup All-purpose flour
• 2 cups half-and-half, room temperature
• 2 cups shredded Gruyere cheese, divided
• 1 tablespoon chopped fresh thyme
• ¾ teaspoon ground black pepper
• chopped fresh thyme, for garnish