Sauté onion, leek and garlic in butter for 3 minutes. Do not allow any colour to form. Add thyme sprigs and white wine and reduce until wine evaporates. Add potatoes and stock and bring to a boil. Cook until potatoes are tender, add watercress and bring just to a simmer. Add cream at this point, if desired. Purée in a blender while still warm, season with salt and pepper, and serve immediately.
Scrape scales from the salmon skin with a knife or fish-scaling tool. Cut fillet into 6 equal-sized pieces. Pinching each piece between your fingers, make two small slices in the salmon skin, no more than 1⁄8-in (3-mm) deep. This will prevent fish from curling in the pan. Heat oil in a non-stick or cast iron frying pan. Add fish, skin side down, and cook for 4 minutes, until skin is crispy and slightly browned. Remove pan from heat and flip fish, allowing the residual heat in the pan to finish the cooking.
Ladle 6 to 8 oz (170 to 225 mL) of broth into a soup plate and place seared salmon in the centre of the bowl, crispy side up. Spoon a small amount of crème fraîche and a teaspoon of caviar on the skin, and garnish with sliced chives or a sprig of fresh watercress.
Serves 6