Microwave the coconut milk in a small microwaveable bowl on high for 20-30 seconds. Add the lemon grass pieces and leave to stand for 10 minutes. Place the coconut milk, lemon grass and remaining coulis ingredients in a blender. Cover and blend for 30 seconds or until the lemon grass is shredded.
Strain the mixture, pressing the solids with a spoon to extract any liquid. Mix the breadcrumbs, Sesame Seeds and Sea Salt on a large plate. Grind generously with Black Pepper.
Moisten the tuna steaks lightly with water and coat the tuna on both sides with the sesame seed mixture, pressing firmly so that the mixture sticks to the tuna.
Heat 1 tbs oil in a large griddle pan on a medium-high heat. For medium-rare tuna, cook for 1 - 1½ minutes on each side. Set the tuna aside and cover loosely with foil to keep warm.
Heat the remaining oil in the same griddle pan on a medium-high heat. Add the pak choi and lychee halves and cook for 1 minute, stirring or just until heated through.
To serve, slice the tuna into thin slices. Arrange the pak choi and lychees in the middle of each serving plate and top with the tuna slices. Season to taste and then drizzle with the Lemon Grass and Lychee Coulis and serve.
