Cut the radishes, parsley and chives into small slices. Brown half the radish.
In a bowl, mix the cheeses, yogurt and mustard. Add the browned radish, lemon juice, sugar, and parsley - salt and pepper to taste. Combine well. Taste and, if necessary, add more salt, pepper, lemon juice or mustard. Reserve it.
Take 4 slices of rye bread. Spread some butter on one side of the slices and place in the oven to heat them. Remove from the oven as soon as the butter has melted.
Remove the avocado peel as whole as possible. Slice the avocado, tomato and chives. Cover the bread with avocado, tomato and salmon, in that order. The number of slices depends on how thin they are - usually one or two slices of each is sufficient. Arrange some of the sauce on top of the salmon and decorate with a slice or two of radish and chives.