Preheat your oven to 350°F / 175°C. Lightly oil a large casserole dish.
Prepare the tortilla strips.
Cut the tortillas into strips about 1 inch wide and 2 inches long.
In a large skillet, heat about an inch of vegetable oil over medium-high heat.
Fry the tortilla strips in batches until crisp and golden.
Remove with a slotted spoon, drain on paper towels, and salt them lightly.
Sauté the mushrooms.
In a separate large skillet, heat the olive oil over medium-high heat.
Add the garlic and sauté until fragrant, about 30 seconds.
Add the sliced mushrooms, tossing until they release their liquid and it evaporates.
Stir in the thyme, oregano, salt, and pepper.
Cook for another minute, then set the mushroom mixture aside.
Create the sauce.
In the same skillet, heat 1½ tablespoons of vegetable oil.
Sauté the green onions for 2-3 minutes until softened.
Stir in the puréed tomatoes, green chilis, and cilantro.
Season with salt and pepper and let it simmer for 5 minutes. Return the cooked mushrooms to the skillet and stir to combine.
Assemble the casserole.
Spread one-third of the fried tortilla strips in the bottom of your prepared casserole dish.
Top with one-third of the grated cheese, followed by one-third of the tomato-mushroom sauce.
Repeat to create two more layers, ending with the sauce.
Add the custard.
In a medium bowl, whisk together the eggs and milk until smooth.
Pour this custard mixture evenly over the entire casserole. You can gently poke a few holes with a knife to help the liquid seep through the layers.
Bake and serve.
Cover the casserole and bake for 30 minutes, or until the custard is set and the cheese is bubbly.
Let it rest for a few minutes before serving.
Enjoy hot with a side salad.
