Preheat the oven to 350 degrees.

Pour the oil into a small skillet over medium heat. When it shimmers, add the shallot, garlic, and jalapeno and cook until soft but not browned. Add the spinach and stir-fry until it has just wilted, then scrape the mixture into a bowl and stir in the yogurt. Season with salt to taste.

Warm the tortillas to make them more pliable. Immediately wrap them in foil to keep them warm.

Pour the thinned-out tomato sauce into the skillet that you sauteed the shallot mixture in and bring it to a boil over medium heat, then reduce the heat to low so that the sauce is barely shimmering. Use tongs to dip the tortillas into the sauce one at a time, leaving them in for just a few seconds; lift them out, letting the excess sauce drip off, and transfer them to a plate.

Spread about a quarter of the sauce on the bottom of a small casserole or individual gratin dish. Lay the softened tortillas on a clean surface. Place half the spinach mixture in the center of each one, then roll the tortillas to form enchiladas and arrange them seam side down on top of the layer of sauce in the casserole dish. Spoon the remaining sauce on top and sprinkle with the grated cheese.

Bake until the cheese has melted and the sauce is bubbling, about 20 minutes. Sprinkle the enchiladas with the cilantro, and eat hot.