Preheat the oven to 180C/350F/Gas 4.
For the stuffed lemon chicken, lay out the three pieces of chicken breast on a sheet of cling film. Cover with another sheet of cling film and flatten the chicken with a rolling pin or meat mallet.
Remove the cling film. Pile the chopped tomatoes and cheese into the centre of each chicken piece. Season with salt and freshly ground black pepper, then roll up the chicken around the stuffing, securing with a cocktail stick.
Heat the oil in an ovenproof frying pan and sear the stuffed chicken on all sides until golden-brown, about 3-4 minutes. Sprinkle over the lemon zest and squeeze over the juice. Transfer to the oven and roast for about 10-12 minutes, or until completely cooked through.
For the citrus spinach, place the spinach into a bowl with the lemon juice, oil, salt and freshly ground black pepper and stir well.
For the pepper sauce, heat the oil in a pan and gently fry the onion, garlic and orange pepper for 4-5 minutes.
Add the chilli flakes, sugar, white wine vinegar and orange juice and simmer for 6-7 minutes, or until the sauce is reduced and sticky and the peppers are soft.
To serve, heat a frying pan until hot and place the lemon halves, cut-side down, in the pan. Leave for 4-5 minutes, until charred.
Place a bed of citrus spinach on a serving plate and top with the stuffed chicken (remove the cocktail sticks). Arrange the chargrilled lemon halves to one side and the pepper sauce in a small bowl on the other side.