Pattypan squash is a variety of summer squash. Round and slightly flattened, this small squash has a scalloped edge. Find it in grocery stores and farmers’ markets.
One medium zucchini, cut into 1-inch pieces, can be used in place of the squash. If you like mushrooms, go ahead and add them for the last 10 minutes of grilling.
If you don't have a grill basket, make sure the vegetable pieces are large enough so they don't fall through the grill rack. Cut the vegetables the same size so they'll cook in the same amount of time. Or remove vegetables from the grill as they become done.