Bacon Pasta With Cheese Sauce and Thyme

Name: Bacon Pasta With Cheese Sauce and Thyme
Ingredients: 8 ounces bacon end strips, or regular bacon, cut into ½-inch pieces (1 ¼ cups); 1 small onion, chopped (1 ½ cups); 4 large cloves garlic, minced (2 tablespoons); 1 teaspoon chopped fresh thyme, plus whole leaves for serving; Kosher salt and freshly ground pepper; 1 pound spaghetti; 2 tablespoons unsalted butter; 2 tablespoons unbleached all-purpose flour; ½ cup whole milk; 8 ounces mixed grated melting cheeses, such as Gruyère, Monterey Jack, and fontina