Prepare a grill for medium-high heat on one side and low heat on the other side.
Prepare the brat hot tub: In a 12-by-10-inch aluminum roasting pan, combine the hard cider, butter and onions. Place on the hot side of the grill.
Grill the brats on the cool side of the grill with the lid closed until the internal temperature of the brats reaches 160 degrees F, 12 to 15 minutes per side. Transfer the brats to the brat hot tub, close the grill and simmer for 10 to 15 more minutes.
Put the sauerkraut and caraway seeds in a small flameproof skillet and place directly on the grill until just warmed through, about 5 minutes. Remove from the heat, add 1 cup of the 18,000 Island Dressing and toss to combine.
For assembly: Nestle a bratwurst and a couple slices of the braised onions into each bun. Smother in warm Swiss Cheese Sauce, then top with some of the kraut mixture. Garnish with sliced chives and thin slices of Fresno pepper if using.
18,000 Island Dressing:
In a food processor, add the mayonnaise, ketchup, jalapenos, Dijon, garlic and some salt. Puree, taste and adjust the seasoning if necessary.
Swiss Cheese Sauce:
Set a medium saucepan over low heat and add the evaporated milk. Toss the Swiss cheese and cornstarch together in a bowl until totally coated. Whisk the Swiss and American cheese into the saucepan and heat, stirring constantly, until the cheese is completely melted, about 5 minutes. Season with salt and pepper.