Add the sausages to a large skillet, along with 1 tablespoon of the olive oil. Bring to medium heat and cook for 5-7 minutes or until lightly browned on each side, turning so all sides are evenly cooked. Remove from the heat, cover, and rest while you cook the peppers and onions.
In the same skillet, heat 2 tablespoons of the olive oil over medium heat. Add the onion and season with 1 teaspoon of salt. Sauté the onion for 10 minutes. If the onions start browning too much, reduce the heat to medium-low.
Add the bell peppers, garlic, remaining ½ teaspoon of salt, and the pepper flakes, and stir to combine. Cook for 10-15 minutes or until the peppers soften but still have a bit of crunch. Add the balsamic vinegar and stir to scrape up any cooked bits from the bottom of the skillet. Add the sausages back to the pan, reduce the heat to low, and cover with a lid so the sausages warm up for a few minutes before serving.
Lightly toast the hoagie rolls in a toaster oven or under a low broiler. Spread with grainy mustard, mayo, pesto mayo, if desired, and load them up with the sausage, peppers, and onions. Top with cheese and place back in the toaster oven or under the broiler to melt the cheese.