Drain and rinse the chickpeas. Place the chickpeas in the bowl of a food processor and puree to a mealy consistency. Add the garlic, cumin and mastiha powder and pulse on and off to combine. Add the tahini and process to a smooth paste, all the while drizzling in olive oil until the hummus achieves a creamy consistency. Balance the flavor with a little lemon juice or vinegar and season to taste with salt. If it is too dense, dilute it with a little water. Set it aside.

Caramelize the onions: Gently heat the olive oil in a large, heavy nonstick skillet and add the onions. Stir to coat in the oil, season with a pinch of salt and cook the onions for about 30 minutes on low heat until soft and amber in color. Stir the onions up every few minutes while they cook to prevent them from burning. About 5 minutes before removing from heat, add the petimezi and/or balsamic.

Spread the hummus into a bowl or platter and garnish with the onions.