Heat oven to 425°F. Grease 13x9-inch pan with shortening. Unroll dough; place in pan. Starting at center, press out dough to edge of pan. Bake 8 to 10 minutes or until light golden brown.
Meanwhile, in 10-inch skillet, cook spinach and chopped onion in butter until onion is crisp-tender. Stir in potatoes; mix well. In small bowl, combine oil and garlic salt; mix well. Brush over partially baked crust; top with spinach mixture. Sprinkle with mozzarella cheese and then Cheddar cheese. Top with onion rings.
Bake 15 to 20 minutes or until edges of crust are golden brown and cheese is melted.