Preheat the grill to medium-high heat (350-450ºF). While the grill is preheating prepare the onions.
For each onion, remove the skin and slice into 1/2 in rings. Keep the slices together as well as possible. Discard the root ends
Using a pastry brush, add avocado oil to either side of your onions. Add salt and pepper to taste.
Add your onion slices to the preheated grill. If the grill grates are fairly close together, place them directly onto the grill. If not, use a grill grate topper or skewers to prevent onion slices from falling through.
Cook onions for 10 min on each side. Flip and cook for another 10 min. Remove from grill and serve.