Slice zucchini into 1/4 inch thick rounds.
Place zucchini and onions on a piece of heavy duty aluminum foil, or a double layer of regular aluminum foil.
Sprinkle with season salt and pepper. Top with butter.
Fold edges over to create an aluminum foil packet. Leave some air space in the packet. Make sure the edges are sealed good.
Place packet on heated grill (I put them on the grill when I put my meat on the grill) Continue cooking zucchini for about 12 minutes. Flip once.
Remove from the grill and leave the packet closed for a couple minutes to let it steam. Be careful when opening the packet because steam will escape.
Top with chopped basil and grated Parmesan cheese if desired
I like to make this when I'm grilling meat. I leave the zucchini packets on while the meat is cooking and they are done at the same time!!