Zest your lemon into the bowl of a food processor or blender. Add the oregano leaves, half the garlic clove, a pinch of salt, 2 tbsp olive oil and 2 tbsp lemon juice.
Whizz into a dressing. Taste. Tweak 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. Rinse well. Halve and thinly slice the fennel. Halve, peel and thinly slice your onion. Put your onion in a sieve. Pour over some boiling water. This will soften and sweeten the onions a little.
Fold the beans, fennel and onion into the tahini dressing. Taste. Add a pinch of chilli flakes, to taste. Finish with the oregano dressing drizzled over and more chilli flakes (if you like) sprinkled over the top.