Peel bananas and onion. Cut the bananas into pieces, dice the onion, pour both into a tall mixing bowl and puree with the chopping stick. Add ketchup. Wash the lemon hot, dab dry and finely grate the peel.
Squeeze the lemon. Stir oil, lemon peel, curry, honey and 1-2 teaspoons of lemon juice into the puree and season to taste. Chill the sauce and garnish with lemon balm just before serving.