Mix the cucumber, shallot and half a teaspoon of salt, put in a colander and leave to drain for 15 minutes.
While the cucumber is steeping, place the lemon juice and garlic in a large bowl and stir to combine. Allow to sit for at least 10 minutes to mellow the harshness of the garlic.
Whisk the yogurt, mint, cumin and cayenne into the lemon-garlic mixture. Add the cucumber and shallot mixture, the chickpeas and the tomatoes, and stir gently.
Spread over the base of a large, shallow bowl. Serve as a dip with warm pita, or spooned over rice or chicken.