Preheat oven to 375 degrees. Spray a 9” x 13” x 2” baking dish with non-stick cooking spray; set aside.
Heat spinach in microwave oven according to package directions, just to thaw it out, not heat it through. Place spinach into a clean kitchen towel and squeeze dry; set aside.
Heat the vegetable oil in a medium non-stick skillet over medium-high heat. Add in the shallots, garlic and 1/2 teaspoon salt and cook until they begin to soften, approximately 2 - 3 minutes. Add in the reserved spinach, mix to combine; set aside.
Place the yogurt into a medium bowl, add 4 teaspoons of the ranch seasoning mix; mix to combine; set aside.
Place the enchilada sauce into a medium bowl, add in the remaining ranch seasoning mix, mix to combine; set aside.
Place the chicken into a large bowl, add in the reserved spinach mixture, 6 ounces of the cheese, remaining 1/2 teaspoon salt, black pepper and 1/2 cup of the ranch yogurt sauce, mix to fully combine; set aside.
Lay the tortillas in a single layer onto work surface. Evenly divide the chicken mixture between the tortillas, placing it down the center of the tortillas. Start rolling up the tortillas, by folding over an empty end of the tortilla over the filling, then rolling up cigar style. Follow this process for all 6 tortillas.
Spread 2 - 3 tablespoons of the enchilada sauce into the bottom of the prepared baking dish. Place rolled tortillas, seam side down, into baking dish. Pour remaining sauce over tortillas, being sure to cover them entirely in sauce. Top with remaining 2 ounces cheese. Cover dish with aluminum foil, place in preheated oven and bake until heated through, approximately 30 - 35 minutes.
Serve ranchiladas with remaining ranch yogurt sauce on the side.