Spray a large pan with low-calorie cooking spray and set on a medium heat. Add the onion and garlic and fry for 5 minutes. Then add the chicken, and continue to fry for 5 minutes, until lightly golden brown on all sides.
Turn the heat up, and after 2 minutes, add the mushrooms and diced asparagus stems. Add the stock cubes to the water, and stir until dissolved. Pour 100ml of the stock into the pan — along with the tomatoes, oregano, chilli flakes, and white wine vinegar. Add the rice, and stir well.
Stir until the liquid has cooked into the rice, then turn the heat down to a simmer. Keep adding ladlefuls of stock and stirring, allowing each ladleful to be absorbed before adding the next.
Carry on adding stock until the rice is soft, but with a slight bite (this will probably take about 20-30 minutes). Just before the rice is cooked, add the asparagus heads.
Immediately pour into a serving dish, then stir in the parmesan and lemon juice, and season to taste with salt and pepper. Cover for 5 minutes or so, to allow all the juices to soak in. When ready, serve!