Bring 6 cups of water to a boil and add 1 tablespoon of salt.
Add the lentils, garlic and sage. Boil the lentils until tender yet still firm, about 20 minutes. Drain in a colander over the sink and place in a mixing bowl.
Combine the lentils with the vinegar and season with salt and pepper. Set aside.
Prick the sausage with a pin several times. Place in a large pot of cold water and bring to a boil over medium heat. Reduce the heat to a very low boil and cover the pot. Cook for 1-1/2 hours.
Place the marinating lentils on a large serving platter to form a bed for the Cotecchino. Remove the Cotecchino from the cooking liquid and drain. Slice the Cotecchino into 1/4-inch rounds and place on a platter over the lentils. Serve.