Remove the outer leaves from the cauliflower, cut off the stalk and roughly chop, then cut the head into small florets. Toss a quarter of the florets in 1 tbsp oil and 1 tsp of the fennel seeds, season well, then tip into a roasting tin and set aside.
Heat oven to 220C/200C fan/gas 7. Heat ½ tbsp oil in a saucepan over a medium heat and add the remaining fennel seeds, toast for 2 mins, then add the lentils and the remaining cauliflower. Stir in the curry paste, then add 1 litre water and bring to the boil. Simmer for 25 mins until the cauliflower is tender and the lentils are cooked through.
Meanwhile, put the roasting tin of cauliflower in the oven and cook for 20 mins until crisp and slightly charred. Tip the soup mixture into a food processor and blitz until smooth, tip back into the pan to warm through, adding the lemon juice and a little water if it’s too thick. Tip into bowls and top with the crispy cauliflower and fennel seeds to serve.