Preheat the oven to 375°F.
Place the onions in a heatproof bowl. In a small saucepan, combine the remaining ingredients and bring to a boil. Pour the hot liquid over the onions to pickle and cool to room temperature.
Toss the potatoes with 2 tablespoons oil, salt, and pepper on a baking sheet. Roast for about 20 minutes, until tender. Cool slightly, then toss with thyme and parsley.
Dry the salmon 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.
Return the skillet to medium heat and add the bacon. Cook, turning once, until crisp, about 5-7 minutes. Transfer to a paper towel-lined plate. When cool, crumble or finely dice.
Pour all but 1 tablespoon of bacon fat out of the skillet and return to medium-low heat with the leeks. Season with salt and cook, stirring, until softened, 5-7 minutes.
Add the potatoes and bacon to the skillet with the leeks and toss to combine, off of the heat.
To make the chive crema, blend the chives, garlic, salt, milk and lemon juice in a blender or with an immersion blender. Place the sour cream in a mixing bowl and fold the chive mixture into the sour cream to combine fully (do not blend the sour cream or it will turn to liquid).
To serve, divide the salmon between plates. Top with leeks and potatoes, pickled red onions and chive crema. Serve with lemon wedges on the side.