Make the pickled red onions: In a small pot, combine the water, apple cider vinegar, salt, sugar, peppercorns, and coriander seeds, and bring to a boil over medium-high heat, stirring until the sugar dissolves.
Turn off the heat and add the sliced onion, making sure they are fully submerged. Let cool to room temperature for 30 minutes before serving.
Make the Thousand Island Dressing: In a medium bowl, whisk together the mayonnaise, ketchup, pickles, garlic, and vinegar.
In a large bowl, season the ground beef generously with salt and pepper. Use your hands to mix until evenly combined.
Divide the beef into 4 8-ounce portions and form into patties. Place on a cutting board. Use your thumb to press a divot into the center of each burger.
Heat the grapeseed oil in a cast iron or stainless steel skillet over medium-high heat until nearly smoking. Add the burger patties and cook for 3-4 minutes, without disturbing, until browned.
Use a spatula to flip the burgers over and place a slice of cheddar on each one. Cook for another 3-4 minutes, until the burgers are cooked through and cheese has melted.
Place on sesame buns and top with the pickled red onions, Thousand Island Dressing, tomato, and lettuce. Serve with French fries.