Cook the lentils in 600 ml of boiling water for 20-30 minutes. In the meantime, peel and slice the carrots and potatoes. Wash the rosemary and dab dry. Pluck all needles except for a little bit for garnishing and chop finely.
Add the potatoes, carrots and rosemary to the lentils 8-10 minutes before the end of the cooking time. Wash the meat, dab dry and season with salt and pepper. Heat the oil in a large ovenproof pan. Add the meat and fry it all around for about 3 minutes.
Finish cooking in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: stage 2) for 6-8 minutes. Season to taste with salt, pepper and sugar. Cut meat into slices and arrange on plates with the lentil vegetables.
Garnish with rosemary.