Heat oven to 200C/180C fan/gas 6. Line a baking sheet with baking parchment. Tip the chickpeas, ½ tsp ground cumin, ½ tsp ground coriander, cayenne pepper, red onion, 1 garlic clove, sesame seeds, baking powder, parsley stalks and ½ tbsp water into a food processor. Blitz until everything is combined but not mushy – you want the falafel to have some texture, rather than being like hummus. Season to taste, then roll into 18 even-sized balls. Flatten each ball into a disc shape and put them on the baking sheet (the more surface area the crisper they will become). Brush them with ½ tbsp oil. Bake for 20 mins, turning halfway, until golden and crisp.
Meanwhile, clean out the food processor. Briefly pulse the cauliflower until you have a ‘couscous’ consistency. Mix the cauliflower with the remaining ground spices, ½ tbsp olive oil and seasoning. Tip onto a baking tray and roast for 10–12 mins, stirring occasionally, until slightly toasted. Once cool, mix through the chopped parsley, mint and half the lemon juice. Season to taste.
Heat the grill to high (this can be done the night before, if you like). Brush the spring onions with 1 tsp oil, season, then grill until soft and charred for around 5 mins, turning halfway.
Meanwhile, blitz the rocket, coriander, remaining garlic, lemon juice and 1 tbsp water together in a food processor until bright green. Put the tahini in a bowl, then gradually whisk in the herby water until the tahini loosens to a sauce consistency. Season to taste.
Divide the green tahini and spring onions between the baked falafel and tabbouleh in bowls.