Finely chop onions, red and green bell peppers, and tomatoes. Rinse black beans, and prepare sausage if using.

Heat oil in a skillet over medium heat. Sauté onions for 2 minutes until translucent, then add bell peppers and cook another 2 minutes.

Add tomatoes, black beans, sausage, and spices, cooking for another 3-4 minutes.

Crack eggs in a bowl, add milk (optional), whisk with salt and pepper until frothy.

In the same skillet, add butter or oil and heat. Pour in egg mixture, cooking for 1-2 minutes until edges set.

Add filling to one side, top with cheese, fold the omelette and cook for another 1-2 minutes.

Slide onto a plate, garnish with cilantro, avocado, and salsa. Serve with lime wedges.