Place a large rimmed baking sheet in oven; preheat oven to 425°F. Toss together onion, bell peppers, fennel, oil, black pepper and salt in a large bowl.
Arrange vegetable mixture in a single layer on preheated baking sheet. Bake in preheated oven until vegetables begin to soften, about 10 minutes.
Remove baking sheet from oven and toss vegetable mixture. Add sausage and garlic to baking sheet. Return to oven, and bake at 425°F until sausage is warmed through and vegetables are tender, about 15 minutes. Remove from oven; sprinkle with cheese, basil and crushed red pepper.
