Put the potatoes in a medium pot, cover with water and bring to a boil over medium-high heat; lower the heat and simmer until fork-tender, 15 to 20 minutes. Add the cauliflower to the pot and cook until fork-tender, 2 to 3 minutes. Drain and let rest until cool enough to handle. Peel off the potato skins (use a clean dish towel to rub off the skins) and cut into chunks. Combine the potatoes, cauliflower and onions in a medium bowl.
Combine 1/2 cup water, the vinegar, sugar, caraway, 1/2 teaspoon salt, the peppercorns and dill in a small pot and bring to a simmer. Strain and pour the hot vinegar mixture over the vegetables, and toss in another 1/4 teaspoon salt. Cover and refrigerate until cold, tossing occasionally, at least 2 hours (up to overnight).
From Food Network Kitchens