Heat the oil in a large heavy skillet over high heat until it is almost smoking. Add the onion, mushrooms, and chiles, and cook, stirring frequently, until the mushrooms and onion are softened and any liquid the mushrooms have given off has evaporated, 5 to 8 minutes.
Add the salt, and add the spinach by the handful, stirring or tossing to wilt it until it all fits in the skillet. Then cover and cook until all the spinach is just wilted, about 1 minute. Season to taste with lime juice and additional salt.
This dish keeps in the refrigerator for up to two days.
Preheat the oven to 300°F.
Warm the sautéed spinach and mushrooms, if necessary, in a small baking dish, covered with foil, 15 to 20 minutes.
Meanwhile, heat the salsa in a saucepan over medium-low heat until heated through, adding some chicken stock or water, if necessary, to maintain a velvety consistency that is thick enough to coat a wooden spoon, but not gloppy.
Soften the tortillas in oil, then fill them with the spinach and mushrooms.
Arrange them next to each other in a baking pan or a heatproof serving dish, and reheat them in the oven for 10 minutes.
Transfer 2 rolled, filled tortillas to each of 4 plates (or keep them in the serving dish) and spoon the salsa generously over them.
Drizzle with the crema, and top with the cheese, onion, and cilantro.
Serve them with rice, beans, or any other side you like.
* These enchiladas can be assembled in a baking dish (before you top them with the sauce) up to four hours ahead and kept covered with plastic wrap in the refrigerator once they’re cool. Cover the enchiladas with foil and reheat them in a 350°F oven for about 20 minutes, and then spoon the warm sauce over them before garnishing and serving.