Season the beef tenderloin with salt and black pepper. Heat olive oil in a skillet over medium-high heat and sear the beef for 3-4 minutes on each side until browned. Remove and set aside.
In the same skillet, sauté chopped onion for 3-4 minutes until soft, then add minced garlic and cook for an additional minute. Turn off the heat.
In the slow cooker, combine beef broth, Worcestershire sauce, Dijon mustard, and the sautéed onion and garlic mixture. Add chopped rosemary and thyme.
Place the seared beef tenderloin on top of the mixture in the slow cooker. Cover and cook on low for 6-8 hours or on high for 3-4 hours until fork-tender.
Remove the beef from the slow cooker and let it rest for 10 minutes. Stir in heavy cream into the remaining liquid and cook on high for an additional 15-20 minutes until the sauce thickens.
Slice the beef tenderloin and serve with the creamy sauce drizzled over the top, garnished with fresh parsley.