Heat oil with butter in a wide pan, sauté the onions and add the carrots. After 5 minutes add the pumpkin and steam for another 5 minutes. Now rinse the lentils with cold water and add them to the pan, also the garlic, the chilli powder and the thyme. Stir everything gently, add the vegetable stock and bring to the boil. Let simmer for 10 minutes, reducing the liquid. Stir gently occasionally.
When the vegetables are cooked, season with lemon juice, salt and pepper and serve garnished with a sprig of thyme.