Make the filling: in a medium saucepan cook the mushrooms, carrot, onion and celery in a small amount of boiling water for about 5 minutes or until tender (keep the pan covered). Drain. Stir in 1.5 tsp of the lemon juice, half the thyme and 1/8 tsp pepper.
Stir in the tomato. Place the chicken between two sheets of plastic wrap and beat each one until they are about 1/8 inch thick. Sprinkle the chicken with 1/8 tsp salt and a dash of pepper. Spoon 1/4 of the filling onto each chicken piece. Fold in the sides and roll up. Secure with wooded cocktail sticks.
Brown the chicken in a little oil in a medium frying pan, turning occasionally. Add the remaining lemon juice and thyme, stock cube and 1/2 cup of water. Cover and simmer for about 15 minutes or until the chicken is no longer pink. Remove the chicken.
Combine the corn flour and milk. Add the mixture to the frying pan. Cook and stir until bubbly then cook for 2 minutes more. Remove the cocktail sticks and cut the chicken into 1/2 inch slices. Arrange on top of the sauce.