Start by preheating your oven to 375°F (190°C). This ensures that your enchiladas will bake evenly and get that perfect golden top.
In a mixing bowl, combine the shredded sharp cheddar, Monterey Jack, and torn Oaxaca cheese. Add in the cooked black beans, steamed spinach, and sliced jalapeño. Mix everything together until well combined.
In a non-stick skillet over medium heat, warm each corn tortilla for about 30 seconds on each side until pliable. This step prevents the tortillas from cracking when you roll them.
Spread a thin layer of enchilada sauce at the bottom of your baking dish. Take a warmed tortilla, spoon a generous amount of the cheese and bean mixture into the center, and roll it up tightly. Place the rolled tortilla seam-side down in the baking dish. Repeat until all tortillas are filled and placed in the dish.
Once all the enchiladas are in the dish, pour the remaining enchilada sauce over the top. Sprinkle any remaining cheese on top for that extra gooey finish.
Cover the baking dish with aluminum foil and bake for 20 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and slightly golden.
Once baked, remove the enchiladas from the oven and let them sit for a few minutes. Garnish with chopped fresh cilantro, diced avocado, and a squeeze of lime before serving.