Meatballs with Balsamic and Caramelized Onion Sauce

Name: Meatballs with Balsamic and Caramelized Onion Sauce
Ingredients: 6 Tbs. (3 fl. oz./90 ml) extra-virgin olive oil; 1 large yellow onion, diced; 2 garlic cloves, minced; 2 cups (2 oz./60 g) lightly packed chopped spinach; Kosher salt and freshly ground pepper; 1 lb. (500 g) ground pork or mild Italian sausage; 1/2 lb. (250 g) ground chicken thigh meat; 1 cup (1 1/2 oz./45 g) panko (Japanese bread crumbs); 1/2 cup (2 oz./60 g) grated Parmesan cheese, plus more for garnish; 1/2 cup (4 oz./125 g) sour cream; 1 egg, lightly beaten; 1 jar Williams Sonoma Balsamic and Caramelized Onion Braising Sauce