How to Make It
In a bowl, whisk 1 cup of the oil with the shallot and dill; season with salt and pepper. Pour two-thirds of the marinade onto a rimmed baking sheet; add the salmon skin side up. Spread the remaining marinade over the skin. Cover. Refrigerate for 2 to 4 hours.
Meanwhile, in a medium bowl, whisk the sour cream with the mustard, lemon zest, lemon juice, vinegar and remaining 1/4 cup of oil; season with salt and pepper. Refrigerate the sauce until chilled, about 15 minutes.
Preheat the broiler and position a rack 6 inches from the heat. Scrape the marinade off the salmon and transfer it skin side down to another large rimmed baking sheet. Season with salt and pepper. Broil the salmon for about 12 minutes, until an instant-read thermometer inserted in the thickest part of the fillet registers 125°. Let the salmon rest for 10 minutes, then transfer it to a platter. Serve with the mustard sauce.