Cook the lentils in boiling water for about 10 minutes. Wash the thyme, dab dry, put 4 stems aside for garnishing. Remove coarse stems from the remaining thyme. Chop the rest coarsely. Cut the sausages into slices.
Heat the oil in a large coated pan. Fry the sausages for 3-4 minutes until golden brown. In the meantime clean and wash the spring onions and cut them diagonally into rings. Wash the apple, dab dry, remove seeds and cut into pieces.
Take out the sausages. Add spring onions, thyme and apples to the pan and toss over medium heat for about 3 minutes. Drain the lentils and rinse with cold water. Deglaze spring onions and apple with balsamic vinegar and stock.
Add lenses and reheat again. Season to taste with salt, pepper and sugar, arrange and garnish with thyme.