One day ahead, make the steak: In a slow cooker, combine the steak, water, tequila, garlic, and 1 serrano chile. Season with salt and pepper, to taste, and cook, on low, until very tender, about 12 hours or overnight. Shred the steak.
Make the enchiladas:
Preheat oven to 350 degrees F.
Combine the tomatillo and tomato sauces in a saucepan over medium heat and cook, stirring, until hot. Stir in 1/4 cup of the cheese.
Heat the black beans in another saucepan.
Combine little of the sauce, a bit of the black beans, a smidgen of the cheese, small morsels of the steak, and hints of the diced canned chiles in each tortilla and roll, keeping the ends untucked. Arrange the rolls in the pans and pour the remaining sauce over the top. Sprinkle with the remaining cheese and remaining chiles.
Cover the pans with foil and bake for 15 minutes. Serve immediately.