Saute onions in oil for 5 min. ( A non-stick or cast iron skillet that's ovenproof is ideal for this.) Add tortilla pieces, chili powder, garlic, cumin, and salt. Toss until the tortilla pieces are wilted. Puree cottage cheese and tomatoes in blender until smooth. Stir gently into tortilla pieces. Sprinkle with grated cheese. Bake at 350 degrees for about 20 min.