Preparing the strained yogurt:
Pour the yogurt into a cheesecloth in a fine-mesh sieve over a bowl. Let it drain overnight in the refrigerator.
Season the firm strained yogurt with pepper, salt, and a dash of olive oil. Put into a piping bag.
Making the caper salsa:
Mix the capers, parsley, shallot, and olive oil. Set aside.
Preparing the grapefruit:
Cut off the top and bottom of the grapefruits. With a sharp knife, cut away the peel all around, including the white pith.
Cut out the segments (à vif) between the membranes.
Save any juice that comes out and mix with the lime juice.
Avocado and radish:
Cut the avocado in half, remove the pit, and peel. Slice thinly. Also, slice the radishes into wafer-thin discs.
Finishing:
Slice the tuna into thin pieces and place 5 to 6 slices per plate.
Place two slices of avocado and a grapefruit segment on each tuna slice.
Pipe small dollops of strained yogurt next to each piece of tuna, make a small well in each dollop, and fill with a spoonful of caper salsa.
Finish with fleur de sel, pepper, the grapefruit-lime juice, lime zest, and radish slices.