Step 1

Preheat oven to 275ºF. Line rimmed baking sheet with parchment paper for easiest cleanup. Remove the salmon from the refrigerator to take some of the chill off.

Step 2

In a small bowl, combine the sour cream, 2 tablespoons of mustard, the dill, capers, caper brine, salt, and pepper. Zest half of the lemon into to the bowl, then cut it in half and squeeze in the juice from half. Mix to combine. The sauce should be thick but drizzle-able; if it's too thick, add water a small splash at a time until it thins out (or more lemon juice if you like).

Step 3

Place the salmon, skin side down, onto lined baking sheet. Pat it dry with a paper towel. Spread a heaping spoonful (about 3 tablespoons) of the mustard sauce in a thin layer all over the salmon.

Step 4

Roast the salmon until it is just barely cooked through, still incredibly moist, and flakes easily with a fork when you remove it (135ºF when checked with an instant read thermometer), 35 to 50 minutes depending on the shape of your salmon.

Step 5

Let the salmon rest for 5 minutes, during which time it will come up another 5ºF.

Step 6

Drizzle the remaining sauce over the salmon or pass it on the side, garnish with the fresh dill sprigs, then serve.