If you can find an heirloom zucchini called Costata Romanesca, you're in for a treat. It's a delicious, dense zucchini distinguished from the usual garden variety by raised ribs that run the length of its body. When cut crosswise, it makes a pretty scalloped slice. Otherwise, use standard zucchini. A bright parsley-caper sauce adds a fresh, slightly salty note to this simple steamed vegetable.