Preheat oven to 375°.
Melt butter in saucepan over med-high heat. Add onions, red and green peppers and jalapenos. Cook and stir for 8-10 minutes until peppers are softened.
Add garlic, chili powder, cumin, salt and pepper. Cook and stir for about a minute.
Sprinkle with flour. Stir and cook 1 minute.
Stir in chicken broth. Turn heat to high...cook and stir until mixture comes to a boil and has thickened.
Stir in Rotel tomatoes, green chilies and sour cream.
In large bowl mix chicken and cilantro together. Pour sauce over the chicken and mix well.
Spray 9x13" baking dish with cooking spray. Break 6-8 tostada shells apart across bottom of baking dish. Spoon on half of the chicken mixture. Sprinkle with about ⅓ of the cheese.
Add another layer of tostada shells, the remaining chicken mixture a third of the cheese.
Make a layer of broken tostada shells on top. Cover with slices of pepper jack cheese and sprinkle with remaining cheddar cheese.
Cover with aluminum foil sprayed with baking spray. Bake for 20 minutes.Uncover and bake for 15 minutes more.Let sit for 10 minutes before serving.