Sauté onion in butter until onion is tender. Add zucchini and continue sautéing until zucchini are cooked but still firm. Add a little more butter as needed. Sprinkle zucchini with chicken seasoned stock base, salt and pepper. In an 8 x 12 inch Pyrex baking dish, layer zucchini-onion mixture, tomatoes and chiles. Cover vegetables evenly with sour cream and sprinkle top with jack cheese. Bake at 350 until heated through and cheese is melted and bubbly, about 20 to 25 minutes. Serves 6
Casserole can be cooked ahead up to the point of adding the layer of sour cream and Jack cheese. Do this at the time you are planning to reheat the casserole.