Heat up a pan on medium with butter and crack your egg gently (easier to keep the yolk in tact but it's fine if it breaks!). After 3 minutes add in the spinach to the side of the pan. Stir spinach to coat in butter. After 4-5 minutes egg should be done.
Toast 2 pieces of bread until golden. Spread butter on one piece of toast and the cheese and then place egg and spinach on top (the warm egg helps the cheese melt). Crack pepper and sea salt on top.