Don’t buy a larger side of salmon; it won’t fit well in the bag. Serve sliced gravlax with lightly toasted rye bread.

3 red onions, peeled and quartered
1 C. kosher salt
3/4 C. sugar
2 C. coarsely chopped fresh dill (both stems and leaves) plus 1 C. minced fresh dill
1 tsp. pepper
1 (3- to 4-pound) whole side of salmon, skinned, excess fat and brown flesh removed, and pinbones removed
1 recipe Onion Relish and/or Cilantro Cream

Process onions in food processor until liquefied, about 4 minutes. Strain mixture through fine-mesh strainer to extract 2 C. liquid, then stir in salt and sugar until dissolved. Stir in coarsely chopped dill and pepper. Place salmon diagonally in 2-gallon zipper-lock bag and add onion mixture. Seal bag, removing any excess air, and lay fish skin side up on rimmed baking sheet. Place second baking sheet on top and set about 7 pounds of weight (such as heavy cans or bricks) on top. Refrigerate weighted fish until very firm, 12 to 18 hours. Remove salmon from bag and pat dry with paper towels, removing any dill stems. Before serving, sprinkle minced dill over top and sides of salmon and slice very thin on bias. Serve with Onion Relish and/or Cilantro Cream. (Salmon can be wrapped in parchment paper, then plastic wrap, and refrigerated for up to 1 week.)

Onion Relish

The onions are cooked slowly to bring out their sweetness while the sugar and vinegar temper the heat of the chiles, but this relish is still potent, so use sparingly.

2 tsp. olive oil
2 white onions (about 1 pound), peeled and chopped
fine
2 serrano or jalapeno chiles, seeded and minced
1 red bell pepper, stemmed, seeded, and chopped fine
1/4 C. sugar
¼ C. white wine vinegar

Heat oil in large sauté pan. Add onions; cook over medium heat until soft but not colored, 7 to 9 minutes. Stir in chiles and bell pepper; continue to cook until all vegetables have softened, about 5 minutes longer. Stir in sugar and vinegar and cook until pan is almost dry, about 5 minutes. Transfer mixture to airtight container and refrigerate until flavors meld, at least 1 hour. Bring to room temperature before serving. (Relish can be refrigerated for up to 1 week.)
Cilantro Cream

1 C. Crème Fraiche or Sour Cream
4 tsp. minced Fresh Cilantro

Combine all ingredients in a small bowl and refrigerate until ready to serve. (Up to 1 day)