Heat the olive oil in a large skillet over medium-high heat.

Add the sliced beef in a single layer and cook, stirring occasionally, until browned on the outside, 3 to 4 minutes.

Transfer the browned beef to a plate and set aside, leaving any juices in the skillet.

Add the diced onion and minced garlic to the skillet and cook, stirring, until the onion is softened, 2 to 3 minutes.

Add the sliced mushrooms and cook, stirring occasionally, until they release their moisture and turn golden, about 5 minutes.

Pour in the beef broth and scrape up any browned bits from the bottom of the skillet with a wooden spoon.

Stir in the heavy cream, Dijon mustard, Worcestershire sauce, salt, and black pepper.

Bring the sauce to a gentle simmer and cook, stirring occasionally, until it thickens slightly, 3 to 4 minutes.

Return the browned beef and any accumulated juices to the skillet and stir to coat in the sauce.

Reduce the heat to medium-low and simmer until the beef is heated through and the sauce is thick and creamy, about 5 minutes.

Warm the zucchini noodles or cauliflower rice in a separate skillet or microwave just until heated through.

Divide the zucchini noodles or cauliflower rice among 4 plates and spoon the creamy beef and mushrooms over the top.

Sprinkle with chopped fresh parsley before serving.