Pour beans into a colander, rinse with water and drain.
Homogenize the ketchup, oil and half a teaspoon of mustard with a whisk as you would for a barbecue marinade. Peel the garlic and chop it into fine cubes. Pour into the marinade and season well with salt. Add about half a level teaspoon full of oregano, a pinch of marjoram and the beans.
Season the mixture again briefly, add salt or ketchup if necessary and let it stand for at least two hours at room temperature. But best of all overnight in the refrigerator.
Bring to room temperature before serving.
Goes well with grilled dishes, roasted meat or simply with snacks.