Sauteè the chopped onions on some oil, then add the spices and stir well for the mixture to become fragrant. Add in the green beans, chopped into 2-3 cm pieces, and the chopped tomatoes (remove the skin if you want). Stir and leave to cook for about 5 min, then pour in the coconut milk and add the crushed garlic. Bring to the boil and leave to simmer, stirring occasionally for about 15 min. You want the beans soft and everything warmed through, the flavors combined. Season with salt and pepper.