To prepare a Spanish squash casserole casseroles recipe, you will first need to prepare the squash. Squash can be purchased at most grocery stores or you can make your own at home. Preheat the oven to 350 degrees F (180 degrees C) and prepare your oven rack to the lowest setting. Peel and slice your squash into 12 inch (1.3 cm) cubes. Place the cubed squash in the bottom of an 8-inch (22-cm) casserole dish. Pour the cream of mushroom soup into the dish and stir to combine. Bake the casserole for 1 hour and 15 minutes or until heated through and bubbly.