Melt the olive oil and the butter together in a large Dutch oven over medium heat. Toss in the chopped onion and Italian sausage and sauté over medium heat until the onion is tender and the sausage is barely pink in the center, about 8 minutes.
Toss in the garlic and continue cooking for an additional 3 minutes. Drain off any excess drippings, leaving about 1 tablespoon in the pan.
Stir in the crushed tomatoes, tomato sauce, tomato paste and the wine.
Add 1-1/2 teaspoons of the dark brown sugar along with the oregano, parsley, basil, bay leaves, red pepper flakes (adding ½ teaspoon if you like things a bit spicier), kosher salt and black pepper.
Reduce the heat to low and partially cover the pan. Simmer for about 1 hour, stirring occasionally. Check for taste, add the remaining brown sugar if needed to smooth out the acidity of the tomatoes and extra seasoning to suit your taste.
Toss with your favorite pasta and top with a sprinkling of Parmesan cheese.