Heat the oil and butter together in a non-stick frying pan, add the onion and fry gently until soft but not coloured.
Add the bacon and fry until golden-brown.
Add the potatoes and cook over a moderate heat until tender and golden-brown. Season, to taste, with salt and freshly ground black pepper.
Place the washed spinach into a bowl.
In another bowl, whisk together one teaspoon of the olive oil and all of the lemon juice. Pour over the spinach and stir well to coat the leaves.
Heat the remaining two teaspoons of olive oil in a small non-stick frying pan. Crack in the egg and fry for 4-5 minutes, or until set. Season, to taste, with salt and freshly ground black pepper.
To serve, place the sautéed potatoes on a serving plate, pile the dressed spinach on top and place the fried egg on top of the spinach.