Combine brine ingredients with 1 litre of water in a saucepan and bring to a boil to dissolve solids.
Cool the brine, submerge the beef for 1 hour, then drain.
Coat beef in olive oil, garlic, chilli, lemon zest, and thyme, then marinate for at least 1 hour.
Preheat the barbecue for direct heat cooking.
Mix the salsa ingredients together, adding lemon juice at the end.
Season beef and grill on high heat for 2 minutes per side until charred.
Move beef to a cooler area to rest for 2 minutes.
Coat onions in oil and seasoning, then grill for 4 minutes until charred.
Slice steak and serve with the onions and salsa.