BROWN meat with onions in large skillet. Add seasoning mix and water; simmer on medium-low heat 10 min., stirring occasionally. Stir in salsa.
SPREAD 1/4 cup enchilada sauce onto bottom of slow cooker sprayed with cooking spray; cover with 4 tortillas, overlapping as necessary to form even layer. Top with 1-1/2 cups meat mixture, 1/4 cup enchilada sauce and 1/2 cup cheese. Repeat all layers twice, using half the remaining meat mixture in each layer.
COOK on LOW 4 to 5 hours (or on HIGH 2 to 3 hours). Serve topped with sour cream and cilantro.