Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking dish with vegetable oil spray.
Place potato cubes into a saucepan of lightly salted water. Bring to a boil, then cook for 15 minutes, or until tender. Drain potatoes, and return them to the pan. Pour in olive oil and milk; whip potatoes until smooth using a hand mixer. Once the potatoes are smooth, use a spoon to stir in the pinto beans, onion, and taco seasoning mix.
Place a small skillet over medium heat, and coat with oil or cooking spray. One at a time, heat tortillas until pliable. Place a heaping spoonful of the potato and bean mixture onto each tortilla, then place a slice of Monterey Jack cheese over the bean mixture. Roll up tortillas and place them seam side down in the prepared baking dish. Spoon enchilada sauce and salsa over the rolled tortillas.
Bake for 20 minutes, uncovered, in the preheated oven. Remove from the oven, and sprinkle Cheddar cheese over the top. Return to the oven for 10 minutes, or until cheese is melted and bubbly.