1Heat oven to 350°F. Cook beef and onion in 10-inch skillet over medium-high heat, stirring occasionally, until brown; drain. Stir in hot water, salsa, milk, uncooked Pasta and Sauce Mix. Heat to boiling, stirring occasionally. Reduce heat; cover and simmer about 7 minutes, stirring occasionally, until pasta is tender.
2Cut tortillas in half. Spread 2 cups beef mixture in ungreased rectangular baking dish, 11x7x1 1/2 inches, or shallow 2-quart casserole; top with 6 tortilla halves and 3/4 cup cheese. Spread with 2 cups beef mixture; top with remaining tortilla halves. Top with remaining beef mixture and cheese.
3Bake uncovered 15 to 20 minutes or until hot and cheese is melted.