Heat 1 tablespoon olive oil in 10-inch oven-safe skillet or cast iron pan over medium-high heat. Add 1/2 cup frozen broccoli florets, 1/2 cup sliced mushrooms, 1/2 cup diced red bell pepper, and 1/4 cup diced red onion. Cook for 5–7 minutes, stirring occasionally, until the vegetables are tender-crisp. Remove the skillet from heat and drain any excess liquid if needed.
In a large mixing bowl, whisk together 6 large eggs, 1/4 cup heavy cream, 1/2 teaspoon salt and 1/4 teaspoon black pepper. Stir in 1 cup shredded mozzarella cheese.
Spread the vegetables out evenly over the bottom of the skillet. Pour the egg mixture directly over the vegetables.
Transfer the skillet to the preheated oven and bake, uncovered, for 10–15 minutes or until the eggs are set in the center and no longer jiggle when shaken.
Let the frittata cool for 5 minutes before slicing and serving.