Preheat oven to 400°F and line a large sheet pan with foil.
Mix granulated onion, garlic powder, smoked paprika, and seasonings in a small bowl.
Toss potatoes and onions with olive oil, salt, pepper, and 1/3 of the seasoning mix. Spread evenly on the sheet pan.
Toss bell peppers and cherry tomatoes with olive oil and another 1/3 of the seasoning; layer over the potatoes and onions.
Arrange sausage on top, brush with olive oil, and sprinkle the remaining seasoning over everything.
Bake for 40 minutes, then switch the oven to broil for 4–5 minutes until lightly charred.
Garnish with parmesan or parsley and serve.