Roast onions and sweet potato by drizzling lightly with olive oil and salt, and placing on a parchment-lined baking sheet in a 475° F oven. Roast until tender and slightly browned (15-20 minutes).
While onions and potatoes are roasting, soak tomatoes in hot water for 5 minutes. Drain and chop finely.
Whisk eggs, milk and salt together until well combined.
In an oven-safe skillet (this 10" skillet is similar to mine) heat a small amount of oil over medium heat.
Place the sweet potato, onion and sun-dried tomato in the skillet and heat for 1-2 minutes.
Pour the egg mixture over the vegetables and cook on stovetop for about five minutes.
Transfer skillet to a 400°F oven.
Bake for 10 minutes or until knife inserted into the middle of the frittata comes out clean.