Slow Cooker Bolognese

Name: Slow Cooker Bolognese
Ingredients: 2 tablespoons unsalted butter; 1 tablespoon extra-virgin olive oil, plus more for drizzling; 1 1/2 cups sweet yellow onion, finely diced; 8 large garlic cloves, finely minced; 1 pound ground beef; 1 pound ground sausage, casings removed; 1 1/2 teaspoons Kosher salt; Freshly ground black pepper; 1/4 cup tomato paste; 2 28-ounce cans (about 1 quart) San Marzano tomatoes, coarsely chopped; 3 sprigs thyme; 1/2 teaspoon dried oregano; 10 large basil leaves, plus more for garnish; 2 bay leaves; 1 pound spaghetti; Parmigiano-Reggiano, for serving