makes one 9- by 13-inch pan
Heat oil in large frying pan over medium-high heat. Add the white onion and cook until softened, but not brown, about 5 minutes. Add the optional garlic, cumin, and oregano and cook until fragrant, about 30 seconds.
Add the squash (and canned tomatoes, if using) and cook for 10–15 minutes, until squash is almost soft. Add the black beans, dried/preserved tomatoes (if using), green chiles, and cook for about 5 minutes. Taste and sprinkle in a couple pinches of salt. Remove from heat.
Melt 4 tablespoons butter in the saucepan over medium-high heat, whisk in the flour, and cook, whisking constantly, for a couple minutes until bubbling and just starting to change color. Gradually whisk in the broth and milk and bring to a boil. Lower heat and simmer, whisking frequently, until sauce is thickened and no floury taste remains, about 10 minutes. Stir in the tomatillo salsa. Remove the sauce from the heat and season with salt and pepper.
Preheat oven to 375° F. Using 1 tablespoon butter, grease a 9x13-inch glass or ceramic baking dish. Set aside 1 cup cheese for topping.
Either roll the enchiladas or stack them flat.
To roll, soften the tortillas by placing the stack in the microwave for 30 seconds or on griddle or in the oven. For each tortilla, fill with about ½ -cup filling and a couple tablespoons of shredded cheese then roll up and place in the dish. Try to snug all 12 in the one pan, but you can always make a little side-car with any extra.
To stack, use 16 tortillas and go for 3 layers of tortilla, filling, cheese, and sauce.
Set pan on top of a rimmed baking sheet to catch any drips and bake for 45 minutes, until filling is bubbling. Sprinkle with the reserved 1 cup cheese and bake for an additional 15 minutes. Remove from oven and let stand for 20 minutes before serving.
Garnish with cilantro leaves and/or jalapeño slices, and pass lime wedges.