Preheat oven to 350 degrees F. Coat an 8-9" round baking dish with cooking spray and set aside.
Heat a medium saute pan over medium-high heat. When hot, add olive oil and swirl to coat pan. Add onions and saute for 3-5 minutes until softened and starting to brown. Add sun-dried tomatoes to the onions and saute another 1-2 minutes. Add 1/4 cup cilantro and saute 1 minute until leaves are softened and fragrant. Remove pan from heat and set aside.
Sprinkle about 2 tablespoons of cheese on the bottom of the prepared baking dish. Spread vegetable mixture on top.
In a medium bowl, whisk together the eggs, egg whites, milk, black beans, 1/4 cup cilantro, and salt and pepper to taste. Pour egg mixture over vegetables in the baking dish. Top with the remaining cheese. Bake for 30-35 minutes, until eggs are set and cheese is browned on top.
While the eggs are cooking, mash avocado with lime juice and a couple of pinches of Kosher salt. Add 2 tablespoons cilantro and mix well to combine.
Serve frittata with corn tortillas, a scoop of guacamole and a side salad.