In a large saucepan, combine chicken, water, 1 clove garlic, the thyme, bay leaf, the 1/2 teaspoon salt, and dash black pepper. Bring to boiling; reduce heat. Cover and simmer about 15 minutes or until chicken is tender and no longer pink.
Drain, reserving 2 cups chicken broth. Strain broth through a double thickness of 100 percent cotton cheesecloth. Chop chicken. Set aside. (Or, use 3 cups cubed, cooked chicken and 2 cups canned chicken broth, if you like.)
In a medium saucepan, cook onion, tomatillos (if you like), and 1 clove garlic in the 2 tablespoons cooking oil until vegetables are tender. Remove from heat; stir in green chili peppers, pimiento, and jalapeno peppers. Set aside.
In a medium skillet, cook tortillas, one at a time, in about 1/4 inch cooking oil until tortillas are crisp, turning once. Drain on paper towels. (Or use 12 packaged tostada shells.)
In another saucepan, melt butter. Stir in flour, chili powder, the 1/4 teaspoon salt, and dash black pepper. Add the reserved 2 cups chicken broth (or the canned broth) all at once. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more; remove from the heat. Stir in sour cream.
In a 13x9x2-inch (3-quart rectangular) baking dish, arrange six of the tortillas, overlapping slightly. Layer with half of the chicken, half of the vegetables, half of the sour cream mixture and half of the mozzarella cheese. Repeat layers.
Cover loosely with foil. Bake in a 350 degree F oven for 35 to 40 minutes or until heated through. Let stand for 10 minutes before cutting into squares to serve. Makes 12 servings.