In a medium glass container or jar, add the sliced fennel, red onion, and mustard seeds. In a medium saucepan, heat the vinegar, 1 cup of water, honey, and season generously with salt (about 1 tsp of kosher salt). Bring to a boil.
Pour the hot pickling liquid over the vegetables and set aside to cool. Once cooled, cover and transfer to the refrigerator overnight.
Preheat the oven to 325°F. Line a rimmed baking sheet with parchment paper. Pat the salmon dry, drizzle with the olive oil, sprinkle both sides with salt and pepper, and place on the baking sheet. Roast until cooked through, 10 to 15 mins, or longer, depending on the thickness of the fillets. Let cool to room temperature.
Arrange the salmon on a platter and top with tangles of pickled fennel and onions, spoonfuls of mustard seeds, and sprigs of fresh dill.
Tip: The salmon may be made in advance and refrigerated overnight.
Garnish with pickles just before serving.