Preheat oven to 350° and grease a square baking pan with cooking spray. In a large bowl, whisk together eggs and milk until very frothy. Season mixture with salt and pepper.
Make "tortillas": In a small nonstick pan over medium heat, melt about a 1/2 tablespoon of butter. Add about 1/4 cup of egg mixture and swirl pan. Cover with a lid and cook for 3 to 5 minutes, until the middle of the eggs are set. Remove from heat and repeat, adding more butter as necessary.
Make filling: In a medium skillet over medium heat, cook bacon. Remove bacon pieces from skillet and drain most of fat, leaving about 2 tablespoons of bacon fat. Add spinach, black beans, and cherry tomatoes and cook, stirring occasionally until the spinach has wilted, about 3 minutes. Season with salt and pepper.
On a cutting board, lay out one egg tortilla. Top center with spinach filling and bacon, then roll up tightly. Repeat with other tortillas and filling.
Spread a thin layer of enchilada sauce in baking pan. Add enchiladas in a single layer. Drizzle more sauce on top, then top with cheddar and Monterey Jack. Bake until cheese has melted, about 15 minutes.
Garnish with cilantro, chopped avocado, and pico de gallo. Serve warm.
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