- Prick sausages in several places using a fork. Cook for 6 minutes in a saucepan of boiling water. Drain. On a work surface, cut half of apple into cubes and the other half into thin slices. Reserve.
- In the same saucepan, heat oil over medium-high heat, and sauté leeks and cubed apple for 3 minutes, or until tender. Add cider vinegar, maple syrup, raisins and caraway seeds.
- Season with salt and pepper to taste. Reduce heat to low; cover and cook for 15 minutes. Reserve.
- Cook sausages in a skillet over medium-high heat for 4 minutes per side. Transfer to a work surface and slice thinly on the diagonal. Lay the 8 slices of toasted rye bread on the same cleaned work surface, and spread 4 slices with Dijon mustard.
- Arrange sliced sausage on top and then sliced cheese; place leek topping onto cheese, dividing evenly, and finish with apple slices. Top with remaining 4 slices.
: Serve leek mixture as a condiment for grilled pork or game meat, or with raclette.