Preheat the oven to 200°C, fan 180°C, gas 6. Put the sausages in an ovenproof baking dish. Add the oil and turn the sausages to coat. Bake for 10 minutes before adding all the vegetables and garlic, and stirring together with some black pepper. Return to the oven to cook for 30 minutes.
Make 500ml stock using the beef stock cube. Mix the hot stock with the mustard and pour over the sausages and vegetables along with the lentils, then stir everything together. Cover carefully with foil and cook for a further 20-25 minutes, taking the foil off for the final 10 minutes.