In a container or zip-top bag, combine the dill, garlic, vinegar, mustard and salmon. Marinate for 4-6 hours.
Preheat the oven to 375°F.
On one baking sheet, toss the potatoes with 2 tablespoons oil and a big pinch of salt and pepper. Roast for 15-20 minutes, until the potatoes are crisp.
On a second baking sheet, toss the pearl onions with 1 tablespoon oil, balsamic and a pinch of salt and pepper. Roast for 12-15 minutes, until softened.
Remove the salmon from the marinade, dry with paper towels and sprinkle with salt and pepper. Heat a large skillet over medium heat with the remaining 1 tablespoon oil. When hot, add the salmon skin side-up. Sear until golden brown, then transfer to a baking sheet and finish cooking in the oven for 8-10 minutes.
In a bowl, whisk together both of the mustards, garlic, heavy cream, vinegar, sour cream, olive oil, lemon zest, lemon juice, salt and pepper until emulsified. Adjust the seasoning to taste.
In a blender, purée the parsley, dill, garlic and vinegar. With the motor running, drizzle in the oil until emulsified. Season to taste with salt.
In a medium bowl, toss the celery and tomatoes with the parsley dressing to coat.
To serve, divide the roasted onions and potatoes between plates. Top with salmon fillets, tomato salad, and mustard dressing.