In a small bowl, mix together basil pesto and lemon.
Season to taste with salt and pepper.
Preheat the BBQ to high.
Lightly oil the steaks, and season with sea salt and cracked pepper.
Grill the steaks to desired doneness, 4 minutes per side for medium.
In the last minute, baste the steaks with a little of the pesto sauce.
Once done, remove from heat and rest for 5 minutes.
Serve topped with the remaining pesto and finely sliced chilli.