Spray cooking spray to coat the bottom and sides of a 9 x 13-inch casserole or glass baking dish. Set aside.
Heat 2 tablespoons of the vegetable oil in a large skillet over medium heat. Ensure the oil is shimmering before adding tortillas the pan. Fry for 30 to 45 seconds, then flip and fry another 5 to 10 seconds. Remove to a plate and continue with the remaining tortillas. Do not brown or crisp the tortillas; they need only to be pliable for rolling.
To the same skillet, add the remaining tablespoon of olive oil and again bring to a shimmer of medium heat. Add in the onions and sauté for 2 to 3 until softened. Add the garlic. Cook until fragrant, about 1 minute.
Add the shredded chicken, half the finely chopped chipotle and adobo sauce, 1/2 cup of chicken stock, and the heavy cream. Bring mixture to a bubble. Add 2 tablespoons of dry ranch seasoning and whisk to incorporate. TASTE! If you desire additional heat, increase the amount of chopped chipotle and adobo sauce in 1 teaspoon increments. Keep in mind you will have chopped chipotles in the cheese sauce, so add sparingly.
In a separate large saucepan, melt butter over medium heat. Sprinkle flour over the surface and cook, whisking for several minutes until the mixture is crumbly in texture and appears golden in color. Mixture will smell nutty, and the raw flour smell will be gone. Whisk in the remaining chicken broth and the other half of the chopped chipotle pepper and adobo sauce. Reduce the heat slightly and whisk in heavy cream, some sour cream, and additional ranch seasoning. Add 3/4 cups of Monterey Jack, 1/2 cup Cheddar, and 1/4 cup smoked Gruyere cheeses and whisk to incorporate. TASTE! If you desire additional heat, increase the chopped chipotle and adobo sauce by 1 teaspoon increments. Keep in mind that you have already added chopped chipotles to the chicken filling, so add sparingly.
Toss all of the remaining shredded cheeses together in a large bowl and place beside your prepared casserole dish. Place a tortilla in one hand and use the other to spoon some chicken mixture in a line right down the center of each tortilla. Top with a pinch of cheese and roll the tortilla around the filling. Place enchiladas seam side down in the baking dish and repeat with the remaining tortillas. Bake the enchiladas for 15 to 18 minutes. If the tortillas smell toasty, with a few browned or darkened spots, you're doing it right.
Remove the casserole from the oven. Pour the cheese sauce over the enchiladas. Sprinkle any remaining shredded cheese over the sauce. Place back into the oven for 5 minutes. Turn your oven's broil feature on and allow the cheese to bubble and slightly brown, about 3 to 5 minutes more.
Garnish with fresh cilantro or chopped green onion.