In a small saucepan, heat the water, sugar, vinegar, and salt. Place the onions in a heatproof bowl and cover with the hot vinegar mixture. Cool at room temperature before using.
Preheat the oven to 450°F.
In a large pot, combine the beef jus ingredients and bring to a simmer. Cook for 20 minutes, strain, and adjust seasoning.
Toss the potatoes on a baking sheet with the oil, salt, pepper, and rosemary. Roast until fork-tender, but not mushy, about 15-18 minutes.
Combine all of the horseradish sour cream ingredients in a small bowl.
To serve, split the hero rolls and spread with sour cream. Fill with roast beef, lettuce and pickled red onions. Serve with roasted potatoes and jus for dipping.