2) For the relish:
Combine all ingredients in a bowl and season with salt and pepper, to taste.
3) For the pickled red onions:
Bring vinegar, water, granulated sugar and salt to a boil in a small saucepan over medium heat. Remove from the heat and let cool for 10 minutes. Put the onions in a medium bowl, pour the vinegar over, cover and refrigerate for at least 4 hours and up to 48 hours before serving.
4) For the burgers:
Heat a griddle or large saute pan over high heat. Add the oil and let it heat until it begins to shimmer.
5) Shape the ground beef with your hands into 4 round patties about 4cm thick and season each burger on both sides with salt and pepper, to taste. Cook the burgers until golden brown on both sides and cooked to medium, about 8 minutes.
6) Put the burgers on the baps and top each with a few tbsps of the queso sauce, relish, onions and chips.