Heat 1 tablespoon of olive oil in a large skillet over medium heat. When the oil shimmers, add the sausages and brown on all sides, about 5-6 minutes total. They do not need to be cooked through at this point. Remove the sausages to a cutting board and let rest.
While the sausage rests, add remaining 1 tablespoon of olive oil to the same skillet. Add peppers and onions and cook until just starting to soften, about 8-10 minutes. Stir in garlic and cook until fragrant, about 30 seconds.
Add crushed tomatoes, brown sugar, parsley flakes, basil, oregano, and red pepper flakes. Use a wooden spoon to scrape any browned bits from the bottom of the pan.
Slice the sausage into thick rounds (about 2 inches each), and then return the sausage to the skillet.
Cover the skillet and simmer until the sausage is cooked through and the vegetables are tender, about 10 minutes. Stir in balsamic vinegar. Taste and season with salt, pepper, and additional brown sugar, if necessary. Garnish with fresh herbs and grated Parmesan cheese, if desired. Serve over pasta or rice, or tuck inside toasted hoagie rolls.