Preheat oven to 450 f.
Add all 4 potatoes to a pot of boiling water. Boil for about 10-12 minutes, until softened, but not fully cooked. Remove from pot. Chop 1 potato, and reserve it for the watercress sauce. Cut the other 3 potatoes in chunks, and coat in butter, and season with salt, pepper, and chopped rosemary.
Add salmon and potatoes to a baking dish. Season the salmon with salt and pepper. Bake for 15-20 minutes, until salmon is fully cooked, and potatoes are softened. Make watercress soup while the salmon and potatoes are baking. After the salmon is fully cooked, add 1½ tablespoons sliced almonds to each salmon fillet, and continue to bake, watching carefully to prevent burning, for another 3-5 minutes, until the almonds are toasted.
In a large saucepot, add olive oil, and minced onion and garlic. Saute over medium high heat for 5-7 minutes, until softened and fragrant.
Add reserved, peeled potato, water and bouillon (or stock), Bring to a boil, and continue to cook for another 5-10 minutes, until the potato is fully cooked. Add the watercress, and cook for another 5 minutes.
Transfer the watercress mixture to a blender, and process until smooth.
Return to pot, add the lemon juice, and season with salt and pepper.
Serve the salmon and potatoes over the watercress sauce. Drizzle lemon juice on each salmon fillet.