Ottolenghi’s cauliflower roasted in chilli butter

2 large whole cauliflowers, with leaves (1.9kg)

2 onions, peeled and cut into eighths

8 red chillies, whole with a vertical slit cut into them

1 lemon, cut into wedges, to serve

Salt

For the chilli butter:

120g unsalted butter, melted (or 120ml olive oil, if you want to keep it vegan)

110ml olive oil

1 ¼ tbsp red bell pepper flakes

2 ½ tsp tomato paste

1 ¼ Urfa chilli flakes

90g rose harissa (adjust according to the brand you are using)

¾ tsp Aleppo chilli flakes (or 1/3 tsp regular chilli flakes)

3 garlic cloves, crushed

1 ½ tsp caster sugar