The tuna should be medium rare, that is, cooked and pale on the outside but just warm and still red in the middle. 

1. Cook the tuna medallions in a non-stick pan on both sides - they should still be pink in the center. 
2. Season with salt and pepper and set aside.
3. Cut three round slices of potato and cook them in olive oil. 

Sauce

1. Proceed as for a béarnaise.
2. Reduce the white wine with the chopped shallot and tarragon. 
3. Add the egg yolk and whisk to a sabayon; incorporate the butter and purple mustard. 
4. Season with salt and pepper and set aside.

Garnish

1. Cut a thin beet slice. 
2. Place the black olive purée on top and cover with another thin beet slice. 

Presentation

1. Place a small bunch of dressed mesclun in the center of the plate. 
2. Place the potato rounds around with a warm tuna medallion on each. 
3. Spoon on the sauce.
4. Place the beet ravioli on the side, using the remaining mesclun as garnish.