Preheat the oven to 190 °C or 375 °F.
Dice the sweet potatoes without peeling them and put them in a large heat-proof skillet or casserole in one layer, if possible. Drizzle with olive oil, minced garlic, rosemary, a pinch of salt and blackpepper. Roast for 30 minutes.
Boil beluga lentils in a covered pot with sufficient amount of water for 20 minutes. If using other types of lentils, increase the cooking time to 30 minutes. Drain and add them to the casserole with sweet potatoes together with diced feta cheese and roast for the remaining time or until done. Enjoy!