In a bowl combine the lemon juice, sesame oil, fresh thyme, salt, pepper, and garlic powder and mix well.
Marinate the tuna filet in the mixture for at least 15 minutes in the fridge. Meanwhile, bring a pot of salted water to a boil. Slice the pak choi in halves and boil for 2 minutes.
Place the marinated Tuna and the pak choi on a sheet pan. Cover the Tuna filet with lemon slices and bake for 5 minutes in the preheated oven. Flip the filet and cook for another 5 minutes.
Remove the sheet pan from the oven and garnish with the capers and chopped parsley. Enjoy!