For the pak choi, heat the little olive oil in a non-stick frying pan and sauté the pak choi for 2-3 minutes.
Add the garlic to the pan with the lime juice and soy sauce. Stir together and heat for a further two minutes. Pour in the stock, and reduce the heat and simmer for 2-3 minutes.
For the tuna, heat the oil in a non-stick frying pan and sear the tuna for two minutes on each side. Remove from the heat.
Spoon the pak choi onto a serving plate and serve the seared tuna on top.