Prepare your grill for medium heat and oil the grate.
In a bowl or mason jar, whisk together the garlic, white wine vinegar, and about 1/2 cup olive oil until well combined.
Toss the squash, onion, and bay leaf together with the remaining 1/4 cup of olive oil on a large rimmed baking sheet. Sprinkle with kosher salt and pepper.
Place the squash slices on the grill, cooking them undisturbed for about 3 minutes. Some nice grill marks should appear.
Flip and cook another 3 minutes until squash is tender, and place back on baking sheet.
To grill the onions, you can either cook directly on the grate or use a grilling tray/basket to prevent slices from falling through. Cook for about 5-7 minutes, turning occasionally, until the edges of the onions start to crisp.
Return onions to baking sheet, add the crumbled feta on top, and drizzle marinade. Let sit for a few minutes before serving.