Preheat the air fryer. Set it to 380°F (193°C). A short preheat helps everything cook evenly and brown better.
Slice the veggies. Cut the bell peppers into 1/2-inch strips and the onion into thin wedges. Keep the slices similar in size so they cook at the same rate.
Toss with seasoning. In a bowl, combine peppers and onions with olive oil, garlic, Italian seasoning, salt, pepper, and red pepper flakes if using. Add a splash of balsamic if you like a hint of tang.
Arrange in the basket. Spread the pepper-onion mix in an even layer in the air fryer basket. Place sausage links on top. This lets the sausage drip flavor into the veggies as it cooks.
Air fry the first side. Cook for 8–10 minutes at 380°F. The sausages should start to brown and the peppers should soften.
Flip and stir. Turn the sausages and toss the peppers and onions to promote even cooking.
Finish cooking. Air fry for another 8–10 minutes, until the sausages are browned and cooked through. Internal temperature should reach 160°F (71°C) for pork or 165°F (74°C) for chicken sausage.
Optional melt. If making sandwiches, split hoagie rolls, add sausage and peppers, top with provolone or mozzarella, and air fry for 1–2 minutes to melt the cheese and toast the bread.
Garnish and serve. Sprinkle with chopped parsley. Serve on rolls, over rice, with pasta, or alongside a green salad.