Cut bread into thick slices and then into cubes. Heat butter bit by bit in a pan and roast the bread cubes in it in portions until golden brown. Leave to cool. Cut mortadella into strips. Crumble the cheese into small pieces. Peel onions. Cut one onion into rings and mix with the other salad ingredients.
Clean, wash and drain the spinach. If necessary, pluck into bite-sized pieces and fold into the bread salad. Finely dice the remaining onion. Mix with vinegar and season with salt and pepper. Fold in the oil. Pour the vinaigrette over the salad and let it steep for a short time. Arrange in small bowls and serve