Heat the butter in a saucepan and add the onions with a pinch of salt. Cook, stirring, until it softened and begins to caramelize, about 5-7 minutes. Add the garlic and red bell pepper and cook for another 2-3 minutes. Mix in the mint. Then add the zest and juice of the lemon, as well as the coriander, and cook, stirring, for another 2-3 minutes.