Preheat the oven to 350°F.
In a large ovenproof skillet over medium heat, heat the olive oil.
Sauté the onion, garlic, and bell pepper until the onion is slightly translucent, about 5 minutes.
In a large bowl, gently whisk the eggs together until just combined.
Stir in the sautéed veggies, sausage, and tomatoes.
Season with salt and pepper.
Pour the egg mixture into the skillet, and allow it to cook until the edges begin to pull away, 3 to 5 minutes.
Transfer the skillet to the oven, and cook until the frittata is set, 15 to 20 minutes.
Serve immediately.