In a microwave safe bowl or plate, add onions and cook for 2-3 minutes to soften.
Coat a large nonstick pan with cooking spray. Add 1 teaspoon olive oil and heat in pan. Add chicken, garlic and cooked onions. Cook over medium-high heat, stirring often until chicken is completely white. Use a spatula to keep turning over while cooking. Stir in 8 tortillas, torn or cut into pieces and cumin. Continue to cook for about 5 minutes, until tortillas are soft. Stir often.
Pour undrained tomatoes, enchilada sauce and tomato sauce over chicken/tortilla mixture. Mix well. Cover pan and continue to cook over low heat, about 5 minutes.
To serve: add about 1½ cups of enchilada mixture to each bowl. Top each with 3 tablespoons cheese. Mix in a bit to melt. Top cheese with ¼ cup tomatoes and ¼ cup lettuce. Finally, top with 1 tablespoon sour cream or yogurt, if desired. Repeat with each serving.
Any leftover enchilada mixture freezes great.