Preheat oven to 425 degrees F. Set salmon on a parchment paper-lined baking sheet. Drizzle and sprinkle salmon with 2 Tbsp. of the oil, lemon juice, salt and pepper. Bake 15 minutes, or until just cooked through. To make sauce, place the remaining 2 Tbsp. oil in small- to medium-sized pot set over medium heat. Add the leeks, shallot and garlic and cook until softened, about 3 minutes. Mix in the flour and tarragon and cook 2 minutes more. Slowly stir in the wine. When the mixture becomes thick, slowly mix in the stock. Bring sauce to a simmer and cook 5 minutes, or until lightly thickened. Mix in the mustard, green onion and cream. Simmer sauce 1 minute more; season with salt and pepper. Arrange fish on individual plates or on serving platter or board and serve sauce alongside.
Tip: You could make the sauce in advance and reheat. Add a little more stock to it if it becomes overly thickened when reheating.
Options: Instead of salmon, bake halibut or sablefish fillets and serve them with the sauce. Instead of shallot, use 1/4 cup finely chopped red onion in the sauce. Instead of green onion, swirl 1 Tbsp. snipped fresh chives into the sauce.