Add 16 oz of liquid to the drip tray and preheat your Kenyon Grill to medium-high.
Whisk together the olive oil, garlic, vinegar, and sugar.
Cut the squash lengthwise into ¼ inch slices, then cut the onion into thick slices.
Place the squash and onion on a plate and drizzle with about ⅓ of the marinade and season generously with salt and pepper.
Place the squash on the grate, grill for 3 minutes or until grill marks appear, turn over and grill for 3 more minutes.
Set squash aside, place the onion on the grill for about 5 minutes, turn frequently until they charred around the edges. Remove and add them to the squash.
Sprinkle everything with feta and drizzle over the remaining marinade, and add in jalapeño slices.