Cut each sausage into 4 equal lengths, then cut each section lengthwise, but not all the way through, so sausage lies flat. Heat a large non-stick skillet over medium heat for 3 minutes. Cook sausages 4 minutes per side or until lightly browned. Remove from skillet and keep warm.
Add oil to skillet and heat until hot. Add pepper, onion and garlic; cook, stirring frequently for 4-5 minutes or until vegetables are tender.
Lay sausage on top of pepper mixture; pour marinara sauce over each sausage and top with a slice of cheese. Arrange spinach over contents of skillet; cover and cook 2-4 minutes or until spinach wilts slightly.
Divide spinach among rolls; top with sausage, peppers and onions.