Preheat oven to 400°. Scrub potatoes with vegetable brush under cold running water. Peel potatoes and cut into chunks. Place potatoes in medium saucepan and add cold water to cover completely. Heat to boiling, then reduce heat to simmer. Cover and cook until tender, about 15 to 20 minutes. Mash with butter and salt. Set aside to cool. In small bowl stir together the cheeses and red pepper flakes. Set aside.
In large skillet over medium heat, heat enough oil to make 1/8 inch depth in skillet. Add tortillas one at a time and fry until soft, about 8 to 10 seconds. Flip and cook other side. Do not let tortillas get crisp around the edges; reducing heat to medium-low if necessary.
After each tortilla is cooked, put on plate and cover with foil to keep warm and pliable. To assemble, place each tortilla on flat surface. Portion 1/3 cup mashed potatoes into the middle of each tortilla and pat lightly to flatten. Sprinkle with 2 tablespoons of the cheese and 1 tablespoon chopped pepper.
Roll each tortilla up and fasten with wooden picks, as if sewing them. Arrange tortillas in baking pan or on a baking sheet, pick side up. Bake until cheese has melted and tortillas start to brown and crisp around the edges, about 15 to 20 minutes. Remove toothpicks and serve with a tablespoon of crumbled feta and a cilantro sprig.
Nutritional Analysis per serving: 207 calories, 6 g protein, 25 g carbohydrates, 9.7 g fat (41% of calories from fat), 15 mg cholesterol, 2.5 g fiber, 260 mg sodium.