Preheat your oven to 200C/Fan 180C/Gas 6. Halve the fennel bulbs and slice into 1cm thick pieces. Peel the onions and slice into small wedges. Scatter both onto a roasting tin and drizzle over 1 tbsp oil and a good pinch of salt and pepper. Toss well so everything is coated, and slide into the oven for 20 mins, till the veg are tender and slightly caramelised at the edges.
Zest your lemon into the bowl of a food processor or blender. Add the oregano leaves, 1/2 the garlic clove, a pinch of salt, 2 tbsp olive oil and 2 tbsp lemon juice.
Whizz into a dressing. Taste. Tweak the seasoning as needed. No processor? Finely chop your oregano and whisk with the remaining ingredients. Put the dressing to one side.
Put the 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 beans through a sieve. Rinse well 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, to taste (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).