Start by cooking the fettuccine in a large pot of boiling water, stirring occasionally until it is al dente. The pasta should have a nice firmness to it. Reserve a bit of the pasta water, just in case the sauce needs some moisture later. Do not forget to drain the pasta once it is done!
Heat the olive oil and one tablespoon of butter in a large skillet over medium heat. Toss in the chicken pieces, seasoned with garlic powder, and cook until they are nicely browned, about 4-5 minutes per side, not cooked through. Avoid overcrowding the pan, as it can steam rather than brown the chicken.
In the same skillet, add the remaining butter and throw in the cremini mushrooms. Cook until they are soft and their moisture has evaporated, about 5-7 minutes. Remember, if they look rubbery, they need more time. Stir occasionally to get even cooking.
Pour in the marsala wine and chicken broth while scraping up any brown bits from the bottom of the pan. Let it simmer for a few minutes, until the liquid reduces slightly. Be careful not to let it boil too harshly, as it might evaporate too fast.
Once the liquid has thickened a bit, stir in the Dijon mustard and heavy cream, creating a silky sauce. If it is too thick, a splash of reserved pasta water can help loosen it up. Just do not skip the stirring or you might end up with lumps.
Combine the cooked fettuccine with the sauce in the skillet, mixing everything well until the pasta is evenly coated. Be gentle—over-mixing can break the pasta, so a light toss is key.
Finish with a sprinkle of salt, pepper, and freshly grated parmesan cheese if you would like. Serve it up right away; pasta waits for no one! Just avoid letting it sit too long, as it can dry out.