Mash potatoes with a masher and mix with onions and the 4 ounces of queso fresco. (if mixture is too thick, add a little milk. mixture should be a little lumpy, but should not be as creamy as mashed potatoes.) Season with salt and pepper.
Warm tortillas in microwave for about 15 seconds.
Spoon potato mixture into tortillas and fry until golden on both sides.
Serve and garnish with lettuce, queso fresco, and salsa.