Peel the onions, clean and wash the leek. Cut both into rings.
Heat 2 tablespoons of oil in a pan on medium heat. Add the onions and leek and cook gently for 5 minutes until softened. Add the tomato paste and cook for 2 minutes, stirring occasionally. Add the honey and stir to combine. Pour in the vinegar and vegetable stock, bring to a boil, then reduce heat to medium-low and simmer for 10 minutes. Season with salt, pepper, and oregano.
Meanwhile, heat 1 tablespoon of oil in a second pan on medium-high heat. Add the sausages and fry for 15-20 minutes, turning occasionally, until cooked through (internal temperature 71 °C / 160 °F).
Arrange the sausages with the leek and onion sauce on a plate and serve with mashed potatoes on the side.