Huevos Rancheros Casserole with Baby Spinach

Name: Huevos Rancheros Casserole with Baby Spinach
Ingredients: 12 corn tortillas; 2 cups shredded Monterey Jack cheese; 1 1/2 cups green tomatillo salsa (or use 2 cans 4 oz. each chopped mild green chilies, undrained, or roasted tomato salsa); 6 oz. baby spinach; 4 eggs; 2 cups low fat buttermilk (or use any variety); 1 cup guacamole (for serving, see tip below for homemade guacamole directions (optional), store-bought or homemade); 1 cup low fat sour cream ( for serving (optional), or use any variety)