Before starting, wash and dry all produce. Lightly oil the grill. While you prep, preheat the grill to 500°F over medium-high heat. Roughly chop half the pickles.Halve buns.Peel, then cut half the onion into 1/4-inch slices (whole onion for 4 ppl).Layer two 12x12-inch pieces of foil. Arrange onions on one side of the foil. Drizzle with 1/2 tbsp oil. (NOTE: For 4 ppl, make 2 pouches, using 1/2 tbsp oil and 2 sheets of foil per pouch.) Season with salt and pepper, then toss to combine.Fold foil in half over onions and pinch edges to seal pouch.

Add ranch and mayo to a large bowl. Season with 1/4 tsp (1/2 tsp) garlic salt and pepper, then whisk to combine.Add cabbage, carrots and chopped pickles to the bowl. Toss to combine.Place in the fridge.

Line a baking sheet with parchment paper.Add beef, 1/2 tsp (1 tsp) garlic salt and 1/4 tsp (1/2 tsp) pepper to a medium bowl, then combine.Form mixture into 2 equal-sized balls (4 balls for 4 ppl).Place on the prepared baking sheet. Using a spatula, flatten balls into 1/4-inch-thick patties. Season with salt.

Place foil pouch with onions on one side of the grill. Close lid and grill until tender, 10-13 min.Meanwhile, add patties to the other side of the grill.Reduce heat to medium, close lid and grill patties until golden-brown on grill side, 4-5 min.Flip patties, then sprinkle cheese over top. Close lid and grill until patties are cooked through and cheese is melted, 4-5 min.**

When patties are almost done, add buns to the top shelf of the grill, cut-side down. Close lid and grill until toasted, 1-2 min. (TIP: Keep an eye on buns so they don't burn!)

Spread ketchup over bottom buns. Stack remaining pickles, then patties and grilled onions on bottom buns. Close with top buns. Divide burgers and pickle slaw between plates.