In a large skillet over medium heat, add a splash of cooking oil. Once hot, add the diced onions and bell peppers. Sauté for 3-4 minutes until they start to soften.
Add the diced zucchini and continue cooking for another 2-3 minutes until tender. Stir in the spinach and black beans, cooking for an additional minute until the spinach wilts.
In a separate bowl, whisk together the eggs, milk, ground cumin, chili powder, salt, and pepper. Pour the egg mixture into the skillet with the veggies, cooking gently until the eggs are scrambled and fully cooked, about 5 minutes. Remove from heat.
Warm the tortillas in a dry skillet or microwave until pliable. Lay each tortilla flat and fill with the veggie-egg mixture.
Top each burrito with a sprinkle of shredded cheese and a few slices of avocado.
Roll tightly, tucking in the ends as you go, to form the burrito. If desired, you can toast the rolled burritos in the skillet for 1-2 minutes on each side to give them a golden finish.
Slice each burrito in half and serve with fresh salsa on the side.
Notes
Feel free to customize with your favorite veggies or add hot sauce for extra flavor.