Lamb Hachis Parmentier

Name: Lamb Hachis Parmentier
Ingredients: 2 tablespoons butter; 2 medium carrots (chopped into half-inch pieces); 2 celery stalks (chopped into half-inch pieces); 1 medium yellow onion (finely chopped); Kosher Salt; 3 garlic cloves (minced); 1 pound ground lamb; 1 sprig rosemary (finely minced); 3/4 teaspoon fennel seeds (roughly chopped); 1/2 cup dry white wine; 3 pounds potatoes (peeled and chopped); Salt; 1/4 cup whole milk; 4 tablespoons butter; 1 garlic clove (peeled and smashed); 1 sprig rosemary; 1 cup grated Gruyère cheese (about 4 oz)