Heat some oil in a large frying pan and fry the sausages. Alternatively, you can bake or grill them in the oven.
Remove them to a bowl, lined with paper kitchen towel. Put a plate on top to keep warm.
Heat a spoonful in olive oil in the same pan and gently sauté the leek for about 5-8 minutes, until tender. Add the garlic and peas, the lemon juice and mustard. Add a small amount of water too.
Cover with lid, turn down the heat and cook for 3-5 minutes.
Add the sausages, a spoonful of vinegar and the herbs. Season with salt and pepper. Stir for a minute or so to warm through.
Serve warm.