1⁄4 cup olive oil
1 lb. Italian sausages, casings removed
1 red onion, minced
4 cloves garlic, minced
1 (28-oz.) can whole peeled tomatoes, crushed by hand
1 1⁄2 tbsp. balsamic vinegar
1⁄4 cup finely chopped basil leaves
Kosher salt and freshly ground black pepper, to taste
1 tsp. fine sea salt
1 1⁄4 cups polenta (not quick-cooking) or yellow cornmeal
1⁄2 cup freshly grated parmesan, plus more for garnish