Add the butter to a medium-sized pot over medium heat. Once mostly melted, add the mushrooms and cook until starting to soften, about 3 to 5 minutes, stirring occasionally. Add the onion and cook until softened, about 5 minutes more, stirring occasionally.
Stir in the garlic and cook for 30 seconds, stirring constantly.
Stir in the flour and cook for 1 minute, stirring constantly.
Add the chicken stock, cream, salt, black pepper, cayenne pepper, and mustard powder, and bring up to a boil. Turn the heat down to simmer and cook 3 minutes.
Stir in the chicken, peas, pimentos, and lemon juice and cook until warm throughout, about 2 minutes.
Serve garnished with fresh parsley along with rice, noodles, biscuits, or puff pastry shells if desired.
