Slice potatoes and put in a baking dish. Drizzle olive oil over the potatoes, and sprinkle salt and pepper on top.
Place spinach on top of potatoes, and then put chicken halves on top of the spinach.
Sprinkle the chicken with a little salt, and then squeeze half the lemon over the chicken breasts. Spread the stone ground mustard on the chicken and then bake for 25 minutes, remove from oven and stir the potatoes around a bit, and then bake for another 20 minutes. Remove from oven, allow to cool for a few minutes and enjoy!