Combine all marinade ingredients in a large bowl or container. Add the salmon and marinate for 2 hours.
To make the gratin, preheat the oven to 400°F. Spray a baking dish with cooking spray.
In a large skillet, heat the oil over medium-low heat. Sauté the garlic and thyme until fragrant, about 1 minute. Add the leeks and cook for about 4 minutes, until soft.
Add the cream and bring to a simmer. Simmer for about 20 minutes. Season with salt and pepper.
Layer the gratin into the baking dish in the following order: potato slices, leek cream, parmesan cheese. Continue until the dish is filled, finishing with cheese on top. Bake until cooked through, about 40-50 minutes.
Place the salmon on a baking sheet, discarding marinade, and roast in the oven for 12-15 minutes, until cooked through.
To make the pea soubise, melt the butter over medium heat in a large skillet. Add the onions and sauté for 5 minutes, until softened but not colored. Add the cream and simmer for 5 minutes. Turn off the heat and stir in the peas and spinach until the spinach is wilted. Cool slightly and transfer to a blender. Purée until smooth, thinning with water as needed.
To make the gremolata, purée all ingredients in a blender until mostly smooth.
To serve, spread English pea soubise on large plates. Place salmon on top, topped with gremolata, snap peas, pea sprouts and radishes. Serve with potato-leek gratin on the side.