Preheat the oven to 400F. Spread 1/4 cup of the enchilada sauce on the bottom of a 13 x 9-inch baking dish.
Cut the zucchini lengthwise, into 1/4-inch thick slices until you have a total of 12 slices about the same size. It’s easiest to do this with a mandolin.
Season both sides of the zucchini with 1/2 tsp salt and pepper, then grill on a grill pan over high heat to help dry out the zucchini, until pliable and grill marks form, but not fully cooked, about 2 minutes on each side.
Heat the oil in a medium skillet over medium-high heat. Sauté onions and garlic on low until soft, about 2 minutes. Add chicken, salt, cilantro, cumin, oregano, chili powder, 1/4 cup of the enchilada sauce, and cook 4 to 5 minutes. Remove from heat.
Spread 3 tablespoons chicken on each slice zucchini. Roll up slices and arrange them each seam side down in the prepared dish.
Top with the remaining enchilada sauce and Mexican cheese blend and tightly cover with foil.
Bake 20 minutes, or until the cheese is hot and melted.
Drizzle with sour cream, if desired and garnish with cilantro.