To make sauce, brown ground beef in a large saucepan or Dutch oven along with the onions, garlic powder, salt and pepper. Drain meat on paper towels. Return meat to saucepan and add broth, tomatoes, chili powder, paprika and cumin. Mix well, breaking up tomatoes with a spoon. Bring to a boil. Reduce heat and simmer uncovered for one hour. In a small bowl, mix cornstarch and water until cornstarch is completely dissolved. Gradually add the cornstarch mixture to the chili sauce, stirring constantly. Continue cooking sauce for about 5 more minutes.
For enchiladas, heat oven to 350 degrees. Heat about 1/2-inch vegetable oil in a small skillet, until hot but not smoking. Quickly fry each tortilla in hot oil to soften, about 2-4 seconds on each side; drain on paper towels. In a large bowl, combine Cheddar cheese and onions. Mix well. Spoon 1/3 cup cheese mixture down the center of each warm tortilla. Roll up and place seam side down into two 11X7-inch baking dishes. Top enchiladas with chili sauce.Cover each dish with foil. Bake for 10 minutes or until hot. Remove foil. Sprinkle with american cheese and continue baking 2 minutes or until cheese melts.