Peel and finely chop the onion. Heat 1 tablespoon of oil in a pan. Sauté the onion until transparent. Add the lentils and sauté briefly. Deglaze with 600 ml stock and simmer over medium heat for 10-12 minutes.
In the meantime, clean the fennel, cut in half lengthwise, hollow out, wash and pat dry. Clean and halve the mushrooms. Clean spring onions, cut diagonally into rings, wash and drain. Coarsely grate the parmesan.
Drain the lentils, collect the stock and measure 200 ml. Heat 1 tablespoon of oil in a pan. Fry the mushrooms until golden brown. Add spring onions and toss briefly. Mix lentils, mushrooms and spring onions.
Season with salt, pepper, vinegar and sugar. Wash oregano, dab dry and garnish all but a few leaves, pluck from the stalks and chop roughly. Mix oregano with the lentil vegetables. Fill fennel halves with lentil vegetables and sprinkle with parmesan.
Place in an ovenproof dish and pour in the measured stock. Bake in a preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 40-50 minutes on the lowest rack until golden brown. Cover if necessary after half the baking time.
Garnish with oregano.