Part 1: Prepare the Salsa Verde (If Making from Scratch)

1. Roast the Vegetables: Preheat your oven’s broiler to high. Place the husked and rinsed tomatillos, chopped white onion, garlic cloves, and jalapeños on a foil-lined baking sheet.
2. Broil to Char: Place the baking sheet under the broiler for 5-7 minutes, until the tops of the vegetables are blistered and blackened in spots. Flip everything over with tongs and broil for another 4-5 minutes until the other side is also charred. This charring process adds a deep, smoky complexity to the sauce.
3. Blend the Sauce: Carefully transfer the roasted vegetables and any juices from the pan to a blender. Add the packed cilantro, fresh lime juice, salt, and optional sugar.
4. Puree Until Smooth: Blend on high speed until the sauce is smooth. Be cautious when blending hot liquids; start on a low speed and gradually increase, or vent the blender lid. Taste and adjust seasoning if necessary, adding more salt or lime juice as needed. Set aside.

Part 2: Create the Hearty Vegetarian Filling

1. Preheat Oven: If you haven’t already, preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
2. Sauté the Aromatics: Heat the olive oil in a large skillet or Dutch oven over medium heat. Add the chopped yellow onion and cook, stirring occasionally, until softened and translucent, about 5-6 minutes. Add the minced garlic, ground cumin, chili powder, and dried oregano. Cook for another minute until fragrant. This step, known as “blooming” the spices, deepens their flavor.
3. Combine the Main Ingredients: Add the rinsed black beans and corn to the skillet. Stir to combine with the aromatics and spices. Cook for 2-3 minutes, until everything is heated through.
4. Wilt the Spinach: Add the chopped spinach to the skillet in batches. Stir continuously until it has wilted down completely, which should only take a couple of minutes. The volume will reduce dramatically.
5. Finish the Filling: Stir in the salt, black pepper, and 1/4 cup of your salsa verde. This adds moisture and a touch of tangy flavor directly into the filling. Remove from heat and set aside.

Part 3: Assemble and Bake the Casserole

1. Prepare the Tortillas: This step is crucial for texture. You have two options:

   • Option A (Best Flavor): Pour the 1/4 cup of vegetable oil into a small skillet over medium-high heat. Once shimmering, use tongs to fry each tortilla for about 10-15 seconds per side. You don’t want them crispy, just softened and pliable. Place them on a paper towel-lined plate to drain excess oil.
   • Option B (Quicker/Healthier): Wrap a stack of tortillas in damp paper towels and microwave for 45-60 seconds until they are warm and flexible.

2. Mix the Cheeses: In a medium bowl, toss together the shredded Monterey Jack and sharp cheddar cheese.
3. Begin Layering: Now for the fun part! Assemble the casserole like a lasagna.

   • First Layer (Sauce): Spread about 1/2 cup of salsa verde evenly over the bottom of your prepared 9×13-inch baking dish. This prevents the first layer of tortillas from sticking.
   • Second Layer (Tortillas): Arrange a single layer of tortillas over the sauce, tearing them as needed to cover the entire surface. You’ll use about 4-5 tortillas per layer.
   • Third Layer (Filling): Spoon about half of the black bean and corn filling evenly over the tortillas.
   • Fourth Layer (Cheese): Sprinkle about one-third of the cheese mixture over the filling.
   • Fifth Layer (Sauce): Drizzle about 1 cup of salsa verde over the cheese.

4. Repeat the Layers: Repeat the process once more: a layer of tortillas, the remaining half of the filling, another third of the cheese, and another cup of salsa verde.
5. Final Top Layer: Add a final layer of tortillas on top. Pour the last of your salsa verde over them, ensuring all the tortillas are coated. Sprinkle the remaining third of the cheese mixture evenly over the entire top.
6. Bake to Perfection: Cover the baking dish with foil and bake in the preheated 375°F oven for 20 minutes.
7. Brown the Top: Remove the foil and continue to bake for another 15-20 minutes, or until the cheese is fully melted, golden brown, and the sauce is bubbly around the edges.
8. Let It Rest: This is a very important step! Let the casserole rest on a wire rack for at least 10-15 minutes before slicing and serving. This allows the layers to set, ensuring you get clean, beautiful slices instead of a soupy mess.