Preheat the oven to 180°C/gas mark 4. Roll the slices of bread with a rolling pin until thin, then spread each with a little of the butter.
Lightly grease a 12-hole muffin tray. Press each slice, buttered-side up, into the muffin tin, folding slightly to fit. Place a small amount of spinach into the base of each.
Crack an egg into each bread cup, then bake for 20 minutes, until the egg whites are set.