Preheat oven to 425°F. Line a large rimmed sheet pan with foil for easy cleanup.
Whisk olive oil, garlic, paprika, Italian seasoning, salt, black pepper, and red pepper flakes in a big bowl.
Add potatoes, onion, and bell pepper. Toss until every piece is lightly coated.
Spread vegetables on the sheet pan in a single layer. Roast for 15 minutes.
Stir in sliced sausage, spread again, and roast 12–15 minutes more until potatoes are tender and sausage browns.
Broil 1–2 minutes for extra crisp edges, watching closely to prevent burning.
Finish with parsley and a squeeze of lemon juice. Toss and serve hot.