In the meantime char onion in the very hit over/broiler or on the stove
Add charred onion into the boiling water along with lemon cut in half
Put the crab legs into the boiling water and cook for approx 6 min per pound of crab or until cooked through, internal temp of 75C (depending on the size of the crab)
Garlic lemon butter
Mix soften butter approx. 100g with zest of whole lemon, juice of half lemon, finely chopped 3 garlic cloves) put into the fridge until ready to use
Assembly:
Gently heat garlic butter in the pot
Remove crab legs from the boiling water onto a large plate, pour all over garlic and lemon butter, serve immediately