For the filling: Add the oil to a large pan with straight sides over medium heat. When it begins to shimmer add the onion, cumin, jalapeños, salt and pepper to taste. Cook, stirring, until everything is tender, about 8 minutes. Add the chicken and diced tomatoes. Toss to mix everything together. Stir in the cheese and remove the pan from the heat.
For the enchiladas and sauce: Coat the bottom and sides of a -9-by-13-inch baking dish with nonstick cooking spray.
Visually divide the filling into 12 portions. Lay out one tortilla on a plate or cutting board and spread a portion of filling across the lower center of the tortilla. Pull the bottom edge of the tortilla up and over the filling and roll up. Place the filled tortilla seam side-down in the dish, perpendicular to the short side of the dish, so you’ll have 2 rows. Continue to roll and place the enchiladas in the dish with 6 in each row. There will be a space between the rows. Fill that space with the jalapeño halves and garlic. Pour the mole sauce over the enchiladas and sprinkle the cheese, if using, down the center of each row of enchiladas.
Bake until the edges are bubbly and the cheese is melted, 25 to 30 minutes.
For serving: If using Graveyard Garnishes (see recipe below), cut a small slit in each enchilada and insert garnishes, upright, in the slits. Serve with a pile of lettuce on the side, and if you like, Greek yogurt and a shake of blood-- I mean hot sauce.
Yield:About 12 garnishes
Preheat the oven to 350 degrees F.
Cut shapes from the tortillas such as ghosts, trees and headstones. Arrange them in one layer on a baking sheet and bake until firm and crispy, 4 to 5 minutes. Set aside to cool.