Take the dough out of the fridge about 20 minutes before processing. Drain the chickpeas in a sieve. Peel garlic and chop finely. Puree the chickpeas, garlic, tahini, cumin and lemon juice in a tall vessel with a chopping stick, add olive oil and season with salt and pepper.
Grease a tart mould (12 x 35 cm). Roll out the dough on a floured work surface approx. (16 x 39 cm) and carefully place it in the tart tin, press the edges firmly and overlap slightly.
Cut off any excess dough. Spread all but 2 tbsp. with hummus for serving and chill for approx. 30 minutes.
Clean, wash and cut the peppers into strips. Peel potatoes and cut into cubes. Heat 1 tablespoon of oil in a frying pan, fry the bell peppers in it for about 2 minutes, take them out, put another 1 tablespoon of oil in the pan and fry the potatoes over a high heat for about 3 minutes, turning them over.
Spread the peppers and potatoes on the tart and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 20 minutes. Peel onions and cut into fine rings.
Heat 1 tablespoon of oil in a frying pan, fry the onions in it, sprinkle with sugar, allow to caramelize and deglaze with vinegar. Remove the tart from the oven and cover with the onions. Add the hummus.