1. Preheat your oven to 375 F (190 C). Heat the olive oil in an oven-safe 10-inch non-stick skillet over medium heat. Add the chopped broccoli, cauliflower, diced yellow onion, and shredded carrots. Saute for 5-7 minutes, stirring occasionally, until the vegetables start to soften.

2. Stir in the minced garlic and chopped red bell pepper. Cook for another 2-3 minutes until the garlic is fragrant and the bell pepper is slightly tender-crisp. Remove the skillet from the heat and spread the vegetables evenly across the bottom.

3. In a medium bowl, whisk together the large eggs, milk, salt, and black pepper until well combined and slightly frothy.

4. Gently fold the crumbled sourdough bread into the egg mixture. The bread will absorb some of the liquid, adding a wonderful texture to the frittata.

5. Pour the egg and sourdough mixture evenly over the sauteed vegetables in the skillet. Sprinkle the shredded cheddar cheese over the top of the frittata.

6. Transfer the skillet to the preheated oven. Bake for 15-20 minutes, or until the frittata is puffed, golden brown around the edges, and set in the center. A knife inserted into the center should come out clean.

7. Carefully remove the skillet from the oven. Let the frittata rest for 5 minutes before slicing and serving. This allows it to set completely and makes for cleaner slices.