Heat oil in 10-inch ovenproof nonstick skillet over medium heat. Add sausage and onion, cook until softened, about 5 - 7 minutes. Stir in garlic and cook, stirring until fragrant, about 30 seconds. Add zucchini, bell pepper, and tomatoes; cook, stirring until vegetables are softened, about 6 minutes longer. Stir in basil, salt, and pepper.
Pour eggs evenly over sausage mixture. Cook, without stirring, until eggs are almost set, about 5 minutes.
Sprinkle frittata with feta. Place skillet 5 inches from heat and broil until frittata is set and top is golden, about 3 minutes longer. Slide frittata onto serving plate and cut into 4 wedges. Serve hot, warm, or at room temperature.