In a large bowl, soak breadcrumbs in milk for 1 to 2 minutes.
Add ground beef, egg, garlic, grated onion, salt, pepper, paprika, and thyme; mix gently to combine.
Form the mixture into medium-sized meatballs.
Heat olive oil in a large skillet over medium heat.
Cook meatballs, turning occasionally, until browned and cooked through, about 8 to 10 minutes; remove and set aside.
In the same skillet, melt butter with olive oil; sauté onion or shallot until soft, about 2 to 3 minutes.
Add garlic and cook until fragrant, about 30 seconds.
Pour in beef broth, scraping the pan to loosen browned bits; simmer 3 to 4 minutes.
Stir in heavy cream, Dijon mustard, Worcestershire sauce, black pepper, and salt; simmer gently until sauce thickens, about 5 to 7 minutes.
Return meatballs to the skillet and coat with sauce; simmer 3 to 5 minutes until heated through.
Serve meatballs topped with creamy sauce, garnished with parsley, over mashed potatoes, rice, or pasta.