Thaw peas at room temperature. Peel onions and cut into cubes. Melt butter in a pot. Sauté onions for 3-5 minutes while stirring. Add peas after 3 minutes.
Fill up with broth, bring to the boil and simmer for about 5 minutes. Wash the mint, shake dry and, except for something to garnish, pluck leaves from the stalks. Chop coarsely and add to the soup. Finely puree the soup with a hand blender.
Season to taste with salt and pepper. Remove skin from salmon. Cut salmon into slices. Pour soup into cups, add salmon as garnish and garnish with mint.