Clean, wash and halve the celery. Cut the halves into slices about 1 cm thick (about 20 half slices). Heat some fat in a large, coated pan and fry the celery slices in two portions while turning them briefly until they start to brown slightly.
Season with salt and pepper and place on the fat pan of the oven. Pour stock and cream into the pan and bring to the boil briefly. Wash parsley, dab dry and chop except for a little bit for garnishing.
Sprinkle celery slices with parsley and cheese. Pour on hot cream mixture. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes. In the meantime, peel and finely dice the onion for the salad.
Wash the cucumber, dab dry and also cut into small cubes. Mix vinegar, onion, some salt and pepper. Finally, fold in the oil. Stir the cucumber into the vinaigrette and set aside.
Wash the tomatoes and, except for one, cut them into slices. Clean, wash and drain or spin-dry lettuce well. Mix lettuce and tomato slices and spread the cucumber-onion vinaigrette over them.
Cut the rest of the tomato into quarters, remove the seeds and cut the quarters into small cubes. Arrange celery slices on a plate, sprinkle with tomato concasseé and serve garnished with parsley. Serve with fresh baguette.