Place the bread in a food processor and blitz to make fine breadcrumbs. You could also use a coarse grater to do this.
Place the breadcrumbs in a medium bowl with the minced beef, chopped onion, garlic, mustard powder and season with salt and pepper. Add the beaten egg a little at a time and mix with clean hands until well combined. The mixture needs to be firm enough to form meatballs so take care not to add too much egg, if it's not needed.
Divide the mixture into 12 equal pieces and form into 12 meatballs. Spray a large non-stick frying pan with low-calorie cooking spray and place over a medium heat. When the pan is hot, add the meatballs and cook for 4-5 minutes until sealed and browned on all sides. Remove from the pan, place on a plate and set aside.
Return the pan to a medium heat (no need to wash first), and add the sliced onion, scraping up any meaty bits in and stirring in. Cook for 10-15 minutes until soft and golden. Add the garlic and cook for a further 1-2 minutes.
Add the mushrooms, stock, Dijon mustard, mustard powder and mixed herbs and stir well. Add the sealed meatballs to the pan, and simmer, uncovered, over a medium heat for 20 minutes, turning the meatballs halfway through. The meatball juices should run clear and there should be no sign of pinkness.
Remove the pan from the heat and stir in the cream cheese until completely blended. If the sauce is too thin for your liking, continue to simmer, uncovered, for a few minutes longer until reduced and thickened slightly. If the sauce is too thick for your liking, add a little water to thin it.
Return to the heat and stir in the frozen peas. Continue to simmer for 3-4 minutes until the peas are tender. Serve with basmati rice or other accompaniment of your choice.