Combine flour, 3 eggs, 1/2 teaspoon salt and 1 tablespoon softened butter in a bowl and knead into a smooth dough. Cover and let rest for about 20 minutes.
Soak bread in water. Peel and mince 1 onion. Heat 1 tablespoon of butter in a pan and saute butter until translucent. Rinse spinach and boil in salted water. Drain, rinse, drain, squeeze and mince. Squeeze bread and add to spinach, sausage meat, marjoram, cooked onion, parsley and remaining egg. Season with salt, pepper, nutmeg and marjoram, to taste.
Divide the dough in half and thinly roll each piece on a lightly floured surface. Cut 12 cm (approximately 4 3/4 inches) squares and add a teaspoon of filling to the center of each. Fold dough over to create triangular ravioli. Press edges to seal. Cook in boiling salted water for 12 minutes.
Peel remaining onions and cut into rings. Heat remaining butter and cook onions until golden brown.
Drain ravioli, arrange on plates, sprinkle with onion rings and serve drizzled with a little melted butter.