In a shallow dish, combine black and white sesame seeds and stir to mix.
Season the tuna with salt and pepper and dredge in the sesame seeds, coating the tuna evenly.
In a non-stick pan, warm the oil until smoking. Place the tuna in the pan and cook until the white sesame seeds begin to turn golden underneath (this should take around 1 minute).
Carefully turn the tune over and cook for another minute.
Transfer the tuna to a cutting board and rest while the courgette is cooked.
In the same pan the tuna was cooked in, turn the heat to medium, add in the butter hearts and, once the butter has melted, add in the sliced red chilli and courgette. Toss and coat in the butter for 1-2 minutes. Separate between 2 serving plates.
Cut the tuna in 1cm thick slices and lay on top of the courgette. Serve immediately.
Notes
Make sure your pan is very hot before cooking, and cook the tuna steaks very quickly to retain the vibrant pink colour in the centre of the meat.