Peel and chop the onions. Heat butter in a large pot. Fry the onions in it. Add frozen peas, 2 l water and stock. Bring to the boil. Simmer covered for about 10 minutes.
Heat salmon in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for about 5 minutes.
Wash mint, shake dry and, except for a little, chop and add to the soup. Puree the soup. Season to taste with salt, pepper and lime juice.
Remove skin from salmon, cut salmon into pieces. Pour soup and salmon into cups. Garnish with rest of mint.