1. Place a large pot of water over a high heat (2 litres) and bring to the boil.
2. Add barley to water and boil for 30 mins, or till tender. Strain and set aside.
3. Toast cumin and cayenne in a pot till fragrant, then add spices to barley and mix through.
4. Season with salt and lemon juice, then set aside.
5. For the vinaigrette, place grapefruit juice into a medium pan over a medium heat, and reduce to 300ml.
6. Whisk in olive oil to form vinaigrette. Set aside.
7. Portion and trim tuna and season with salt and olive oil.
8. Sear in hot pan (about 1 min each side).
9. Baste tuna with grapefruit vinaigrette (10seconds each side). Remove from pan and place on board.
1. Dress barley with grapefruit vinaigrette and olive oil. Slice tuna and drizzle with excess vinaigrette from pan.
2. Spoon mounds of barley on board, then finish with seasoning, olive oil and rocket cress.