Peel the onions and cut into thin rings.
Bring the jam, balsamic vinegar and 2 tablespoons water to a boil in a small pot. Add the onions and simmer for about 1 minute over medium heat. Season with salt and pepper and let cool.
Pluck 1/2 the thyme leaves and stir into the sausage.
Toast the bread until golden brown on both sides then spread the liverwurst mixture over top. Sprinkle with the onions and garnish with the remaining thyme.