Melted Mozzarella Casserole with Mushrooms and Smoky Chipotle Chile (Queso Fundido)

Ingredients

• 1 1/2 tablespoons olive oil, preferably extra-virgin
• 1 medium red onion, sliced
• 6 to 8 ounces full-flavored mushrooms, such as shiitakes, oysters or practically any wild variety, stemmed (discard woody stems or finely chop them) and sliced (you'll have about 2 generous cups)
• 2 to 3 canned chipotle chiles en adobo, seeded and thinly sliced
• Salt
• 12 corn tortillas, the fresher the better (store-bought are okay, but homemade will really shine here)
• 8 ounces mozzarella, preferably whole-milk (but not "fresh" mozzarella, which will not melt) or Mexican quesillo, shredded (you'll have about 2 cups)
• A generous 1/2 teaspoon chopped fresh thyme or generous 1 teaspoon chopped fresh epazote
• A little black pepper, preferably freshly ground

Directions

The mushroom mixture: Heat the oven to 350 degrees F. In a large skillet (preferably nonstick), heat the oil over medium-high. Add the sliced onion and cook, stirring frequently, until softening and beginning to brown, about 5 minutes. Add the mushrooms and stir nearly constantly until they have softened and any juice they release has evaporated, about 5 minutes longer. Stir in the sliced chiles, then taste and season with salt, usually about 1/4 teaspoon. Transfer the mixture to a 9- or 10-inch shallow baking dish, Mexican cazuela or pie plate.

Finishing the queso fundido: Very lightly dampen a clean kitchen towel. Check the tortillas to make sure none are stuck together. Wrap them in the towel, then in foil, sealing the edges tightly. Place in the oven and set the timer for 7 minutes. When the timer goes off, stir the shredded cheese into the warm mushroom mixture. Set in the oven alongside the tortillas and bake until the cheese is just melted, about 5 minutes longer. Sprinkle with the thyme or epazote and black pepper and serve without a moment's hesitation, accompanied by warm tortillas.