Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion, garlic and mushrooms and cook for 5-10 minutes, until softening and lightly golden.
Add the vinegar, Worcestershire sauce or Henderson's relish, mustard and stock. Stir well and reduce the heat to low. Simmer, uncovered for about 10-15 minutes, or until reduced by half.
Add the well-drained prawns, asparagus and tomatoes. Simmer uncovered over a medium heat for 4-5 minutes, stirring until the prawns, asparagus and tomatoes are cooked. The prawns should be pink on the outside and white inside. The asparagus should be just tender, and the tomatoes softened.
Remove the frying pan from the heat and stir in the cream cheese until completely blended. If the sauce is too thick, add a little water; if it’s too thin, simmer gently for a little longer.