Preheat your oven to 200°C/Fan 180°C/Gas 6. Halve the fennel bulbs lengthways and slice into 1cm-thick pieces. Peel the onions and slice into small wedges. Scatter both into a roasting tin and drizzle over 1 tbsp oil and a good pinch of salt and pepper. Toss so everything is coated, and slide into the oven for 20-25 mins, till the veg are tender and slightly caramelised at the edges.
While the veg roast, zest the lemon into the bowl of a small blender. Add the oregano leaves, half a garlic clove (peeled), a pinch of salt and 2 tbsp each olive oil and lemon juice. Whizz into a dressing. Taste. Tweak the seasoning. No blender? Finely chop the oregano and whisk with the other ingredients. Set the dressing to one side.
Put 1 tbsp tahini in a large bowl. Whisk with a pinch of salt, 1 tbsp lemon juice and 2 tbsp water. Taste. Add a little of the remaining garlic or more lemon juice, if you like.
Drain the cannellini beans, rinse them with cold water and shake dry.
Fold the beans and the roasted fennel and onion into the tahini dressing. Taste. Add a pinch of chilli flakes (they’re spicy so use as much or as little as you like). Finish with the oregano dressing drizzled over the top and scatter over more chilli flakes, if you like.