Don't have a written recipe. Although I'm sure there is no shortage of Chicken Dijon recipes out there.
I used about 2 pounds of boneless chicken breast of equal thickness. You can always beat your chicken with a mallet on the thick side to equal it out. You are beating your chicken not choking it 
Dredge them in a good coating of flour salt and pepper and brown them in a skillet on both sides. Remove to a plate and set aside.
To the skillet add 2-3 minced shallots. If anyone's knife skills are poor you can throw the shallots in a mini chopper. 
Saute shallots for a few minutes and add a pound of sliced button mushrooms and 3 tablespoons of butter. Cook mushrooms slowly until they develop some color. Add about a cup of good quality chicken stock(homemade or boxed) and raise heat to a simmer.
Add and whisk in a cup of heavy cream and continue to simmer. Add chicken breast back in the pan and continue to simmer on low for a few more minutes. Stir in some Dijon mustard. I use 2-3 tablespoons. But you can add as little as a teaspoon or as much as you like mustard.
The full fat heavy cream will prevent the sauce from separating. Simmer a bit longer if sauce is a bit thin. Season with a bit more salt and pepper. 
I made steamed Jasmine rice and roasted broccoli crowns with garlic olive oil and lemon. Was delish IMHO. 
BTW any mushroom you might like can be used.