Place tomatoes and their juices in a medium pot. Using your hands or a potato masher, break up tomatoes into bite-sized pieces. Add onion, basil, and butter to pot. Bring to a simmer over medium-low heat. Reduce heat to low, partially cover, and cook, stirring occasionally, until onions are very soft, about 40 minutes. Discard onions and basil.

Make Ahead: Sauce can be made 3 days ahead. Let cool, then store in an airtight container and refrigerate.

Preheat oven to 425°. Line a baking sheet with foil and lightly spray with cooking spray. In a large bowl, combine egg, garlic, bread crumbs, Parmesan, milk, parsley, salt, and pepper. Crumble sausage and pork over egg mixture. Using clean hands, mix until just combined. Form pork mixture into 2" balls (about 3 Tbsp. each) and arrange on prepared sheet.

Bake meatballs, stirring after 10 minutes, until lightly browned, about 15 minutes.

Transfer meatballs to sauce. Cover and simmer over low heat, turning occasionally, until flavors have melded and meatballs are cooked through, about 10 minutes. Remove from heat.

Season sauce with salt and pepper. Cover pot while polenta cooks.

Meanwhile, in a large pot over medium-high heat, combine milk, salt, and 2 c. water. While whisking, slowly add polenta in a steady stream. Bring to a boil, then reduce heat to low and continue to cook, whisking vigorously, until polenta is thickened, about 3 minutes. Remove from heat. Stir in mascarpone and Parmesan; season with pepper.

Spread polenta on a platter. Spoon meatballs and sauce over. Top with basil and Parmesan.