Pre-heat the oven to 180ºC.
Spray a saucepan with low calorie cooking spray and sauté the onions over a medium to high heat for 5-6 minutes until golden brown.
Add the tomatoes, garlic powder and stock, stir and bring to a boil. Reduce the heat and allow it to simmer for 15 minutes.
While the Margherita sauce simmers, spray a non-stick frying pan with low calorie cooking spray and place on a medium to high heat.
Place the chicken breasts in a hot pan and seal for about 2 minutes each side, until golden, then arrange them in an ovenproof dish.
After 15 minutes simmering, stir the chopped basil into the sauce and season with black pepper. Spoon over the chicken breasts.
Tear the mozzarella into pieces and scatter over the top.
Place into the oven and bake for 25 minutes until the chicken is cooked through and the cheese is melted and golden.
Scatter a few basil leaves on top and serve with your choice of accompaniment.