Cook the onions by placing in a microwave-safe plate or bowl and cooking for 1-2 minutes, to soften. Next, in a medium or large saucepan, heat oil. Add sausage pieces, cooked onions, and garlic. Cook for 2 minutes. Stir in crushed tomatoes, sugar, Italian seasonings, red pepper flakes, and a little salt and pepper. Bring to a light simmer. Cover and continue to simmer for 10 minutes.