Preheat the oven to 400 degrees . On a work surface, cut the puff pastry into rounds to fit four 1 1/2-cup ramekins or small bowls; transfer to a parchment-lined baking sheet. Prick with a fork, then brush with 1 1/2 tablespoons cream and bake until golden, about 15 minutes.
Meanwhile, arrange the green beans on a foil-lined baking sheet, dot with 1 tablespoon butter and season with salt and pepper. Drizzle 2 tablespoons water over the beans, then wrap into a flat package. Transfer to the oven and roast for 15 minutes, then unwrap, stir and roast until crisp-tender, about 5 minutes.
In a large nonstick skillet, melt 1 tablespoon butter over medium heat. Add the leek, season with salt and pepper and cook until soft, 2 to 3 minutes; transfer to a plate. Add 1 tablespoon butter and the mushrooms to the pan, season with salt and pepper and cook until golden, about 5 minutes. Add the remaining 1 tablespoon butter and the flour and cook, stirring, for 1 minute. Stir in the remaining cream, 1 1/4 cups water and the tarragon; bring to a boil and cook, stirring, for 3 minutes. Season the salmon with salt and pepper; stir into the pan along with the leek. Cook for 3 to 4 minutes, then remove from the heat.
Divide the salmon mixture among the 4 ramekins and top each with a puff pastry round. Serve with the green beans.