Heat oven to 220C/200C fan/gas 7 and line a large baking sheet with baking parchment. Put the onion in a bowl and pour over boiling water to cover. Leave to soften for 15 mins. Meanwhile, beat the cheese, spinach and nutmeg together with plenty of black pepper. Spread over the chicken breasts, then top with the tomatoes.
Drain the onion slices, then toss them with the potato, garlic, olives, oil and black pepper. Arrange the potato mixture on the parchment in 4 piles spaced apart, then flatten. Bake for 25 mins more until almost tender and starting to colour.
Remove from the oven, put 1 chicken breast on each pile of potatoes, and bake for 20 mins until the chicken and potatoes are tender and cooked all the way through, and the tomatoes are roasted. Serve with a big salad.