Cook 2 bunches trimmed broccolini in a large pot of salted boiling water, 3 minutes. Drain; rinse under cold water, then pat dry. Whisk 1/4 cup sour cream, 1 tablespoon each water and whole-grain mustard, 1 teaspoon apple cider vinegar and 1 tablespoon minced shallot; season with salt and pepper.Drizzle over the broccolini.