Preheat the oven to 425 degrees F.
In a large bowl, whisk the eggs, milk, kosher salt and black pepper.
Heat the oil in a 10-inch non-stick, oven-safe skillet over medium heat. Sauté the jalapeños, garlic and 1/3 cup chopped onions for 1-2 minutes until fragrant.
Add the black beans and peppers to the skillet, folding in the spinach and cheese.
Pour in the egg mixture. Give the frittata 2-3 minutes on the stove for the edges to start to set.
Bake the frittata in the oven until it has set in the middle, about 12-15 minutes.
To make the pico de gallo, stir the tomatoes, 2 tablespoons chopped onions, jalapeños, lime juice, cilantro and a pinch of salt in a small bowl.
Cut into wedges and serve with pico de gallo and top with avocados and cilantro.