Vegetable Frittata with Asiago Cheese

1 tablespoon olive oil
1 cup red onion, chopped
1 cup red bell pepper, coarsely chopped
1 cup zucchini, chopped
2 cups spinach leaves, torn into 1-inch pieces, packed
1/2 teaspoon salt
1/4 teaspoon ground black pepper
3 large eggs
6 large egg whites
1 cup (4 ounces) Wisconsin Asiago Cheese, shredded - divided use
1/2 cup tomato, chopped
1 tablespoon fresh basil, chopped

1. Preheat broiler.
2. Heat olive oil in oven-safe, 10-inch skillet over medium-high heat. Add onion and bell pepper; sauté until golden, about 8 minutes. Add zucchini; sauté until tender, about 5 minutes. Add spinach; stir until wilted, about 1 minute. Season with salt and pepper; remove from heat.
3. Whisk eggs and egg whites in medium bowl.
4. Sprinkle 3/4 cup of Wisconsin Asiago cheese over hot vegetables in skillet and toss to combine.
5. Add egg mixture to skillet and stir gently to combine. Reduce heat to medium-low. Cook without stirring until eggs are set on bottom, about 5 minutes.
6. Sprinkle remaining 1/4 cup Asiago cheese over frittata. Broil until cheese melts and frittata puffs around edges, 2 to 3 minutes. Sprinkle with tomatoes and basil.