Pre-heat oven to Gas 6, 200°C, fan 180°C.
Finely dice the onion and mushroom. Heat 1tbsp oil in a small frying pan and fry them until the onion has softened. In a small bowl, mix the onions and mushrooms with the herbs, seasoning, cheeses and prawns.
Carefully slice the chicken breast fillets so that they open out. Lay them flat on a chopping board and place one third of the mix in the middle of each. Place the remaining filling in the fridge. Carefully close the chicken breasts around the filling and secure with cocktail sticks to ensure the filling stays inside.
Place chicken parcels in the frying pan over a high heat to brown, adding a little more oil if necessary and turning until all sides are golden and sealed - about 5-6 minutes. When brown, place them in an ovenproof dish and cook for 15-20 minutes in the oven.
Dissolve stock cube in 200ml (7fl oz) boiling water, add the reserved filling, mix and add to the baking dish. Return to the oven for a further 5minutes.
Serve with new potatoes and green beans.
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