Drain and rinse the butter and kidney beans. Place them in a large saucepan filled with water. Bring to the boil, then reduce the heat and simmer for about 40 minutes until tender. Drain and refresh under cold running water, then remove any loose skins.
Heat the oil and butter in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onions, garlic and sugar and cook for 2–3 minutes to soften. Add the cumin seeds and peppers and cook for a further 1–2 minutes, then add the drained beans and stir to coat well.
Stir in the harissa and the tomatoes and cook over a gentle heat for 30 minutes. Season to taste with salt and pepper, then stir in half the mint and garnish with the remaining mint. Serve with chunks of bread and a dollop of creamy yogurt, if liked, or as an accompaniment to meat and poultry tagines.