In a large mixing bowl, combine the fromage frais, lime zest, half the lime juice and horseradish. Add the cucumber to this mixture and stir well.
Next comes the turn of the mackerel. However, first check for any bones in it and add only after clearing them. Add in the cannellni beans and stir so that everything blends well.
In another mixing bowl, toss together the salad leaves with onions and the remaining lime juice. Arrange these on a salad platter and scoop up the mackerel mixture on them.
Top the mixture up with beetroot and garnish with the lime wedges.
Serve immediately. Cherish the looks of glee on the guest's faces when they savor it.