Rinse, spin dry and trim spinach. Cook spinach in boiling salted water for a few minutes, drain, rinse with cold water and squeeze. Chop coarsely. Peel and chop onions. Heat butter in a pan and saute onions until translucent. Add spinach and saute for another 5 minutes, stirring occasionally. Season with salt and pepper.
Cut off bread crusts and line bottom and sides of a bowl (1 liter capacity) (approximately 4 cups) with bread slices. Mix cheese with spinach mixture. Fill bread bowl with spinach and cheese mixture, cover and weigh down with a plate and a weight. Refrigerate for 4 hours and invert onto a serving plate. Serve.