Pretty much any summer vegetable is delicious on the grill. This combination of green beans, sweet onions, and zucchini or summer squash is a particular blend of snappy, tasty, tender, and overall deliciousness.
The trick to this dish is keeping the green beans on the grill. A smaller-than-normal spacing on your cooking grate will help. A grill basket works, too. Or, you can accept that you're going to lose a few to the fire and wing it.
Prepare a grill for high heat—you want to sear the vegetables quickly. Cut the stem end off an onion or two, cut the onions into wedges, and peel each wedge&leaving the root end more or less intact to hold the wedges somewhat together. Toss the onion wedges with a bit of olive oil or vegetable oil and set aside. Turn them once they are charred on one side, cooking them until tender and browned, about 8 minutes total.
Trim the ends on a few zucchini or summer squash and cut them in half lengthwise. Brush with oil and set on the grill. Cook until tender and browned, about 5 minutes.
Toss cleaned and trimmed green beans with a bit of olive oil or vegetable oil, just to coat them lightly. Put the oiled green beans on the cooking grate (placing them in a grill basket first, if you're lucky enough to have such an implement). If you're not using a grill basket, be sure to lay the green beans perpendicular to the bars of the cooking grate so they down all fall through into the fire. Cook until seared on that side and/or the ends, just a minute or two, use tongs to turn then carefully (this is where you'll definitely lose a few if you don't have a grill basket), and cook them on the other side for another minute or so.
Remove all vegetables from the grill, cut the zucchini into inch-long pieces, put the vegetables on a serving platter, toss together if you like, sprinkle with salt to taste, and serve hot, warm, or even at room temperature. These are great to make for a crowd, and extra delicious next to a grilled steak or grilled chicken.