Peel the zest of the tangerine using a zester, set a little aside, roughly chop the rest. Squeeze the juice out of the tangerine. Puree the salmon and chopped zest with the mustard and 1 tbsp of the tangerine juice. Stir in the cream cheese and tarragon, cover and chill for approx. 30 mins.
Puree the avocado with the cream cheese and lemon juice, season, cover and chill for approx. 15 mins.
Bring the vinegar to the boil with the sugar and salt, pour the hot liquid over the shallots, leave to cool.
Toast the pumpernickel in a frying pan (without oil) for approx.1 min. on each side. Spread the salmon mousse over half of the pumpernickel, garnish with a little tarragon, fried onions and the zest of the tangerine set aside. Spread the avocado cream over the remainder of the pumpernickel, garnish with the shallots and chervil.