Chicken Gratin with Celery, Mushrooms and Brie

Cut chicken breasts into bite-sized cubes. Rinse celery and cut into 4 cm (approximately 1 1/2 inch) long pieces. Peel onions and chop finely. Clean mushrooms, halve or quarter them. Heat butter in a pan and saute onions until translucent. Add chicken and saute for a few minutes. Add celery and mushrooms, saute briefly. Crush peeled garlic and add with lemon juice, broth and cream to the pan. Simmer for 5 minutes and season with salt and pepper. Rinse parsley, shake dry, pluck off leaves and set some aside for garnishing. Chop the rest and add to the mixture. Place everything into a baking dish. Cut cheese into slices and place on top of the gratin.