Prepare your grill by heating it to medium-high, around 400°F (200°C). Allow it to preheat for about 10-15 minutes.

In a food processor, pulse the fresh tomatoes and red onion until finely chopped but still slightly chunky. Let the mixture drain in a strainer for 2 minutes. Combine with apple cider vinegar, honey, kosher salt, pepper, celery salt, red pepper flakes, and parsley.

Soak the two slices of soft white bread in ¼ cup of whole milk for about 2 minutes until softened. Squeeze out excess milk and crumble into a large mixing bowl. Combine with ground beef, beaten egg, finely chopped onion, and a pinch of salt and pepper. Mix gently until just combined.

Divide the burger mixture into four equal portions and shape into patties about ¾ inch thick. Make a small indentation in the center of each patty.

Place patties on the preheated grill. Cook for 3-4 minutes on each side for medium-rare. Adjust the time as necessary for desired doneness.

Transfer the cooked burgers to toasted potato rolls and top generously with the fresh tomato relish.