Bring 6 quarts of water to a boil and add 2 tablespoons salt. Slice tuna into 4 equal pieces and place each between lightly oiled pieces of aluminum foil. Lightly pat tuna with a cleaver until paper thin. Carefully flip each piece onto a chilled plate and refrigerate, wrapped in plastic.

Place sweet potato cubes into boiling water and cook until tender yet firm (3 to 4 minutes). Rinse under running water until cool and set aside.

Remove seeds from cucumber and cut into 1/8th-inch thick half moons with skin on.

Place onion, saffron and white wine vinegar to a boil and reduce by half. Place vinegar mixture in blender with mustard and blend until smooth. Drizzle in a half cup extra virgin olive oil until emulsified and season with salt and pepper. Set aside.

Remove tuna plates from refrigerator and season with salt and pepper. In a mixing bowl, dress cucumbers and sweet potatoes together with 2 tablespoons extra virgin olive oil and season with salt and pepper. Divide sweet potato mixture in center of each plate, drizzle with saffron vinaigrette and serve immediately.