Step 1: Preheat your oven to 375°F (190°C).
Step 2: In a large bowl, combine ground chicken, mushrooms, breadcrumbs, Parmesan, egg, garlic, parsley, onion powder, salt, and pepper. Mix until the ingredients are well incorporated.
Step 3: Shape the mixture into 1-inch meatballs and place them on a parchment-lined baking sheet.
Step 4: Heat olive oil in a skillet over medium heat. Sear the meatballs on all sides until golden brown, approximately 2-3 minutes per side.
Step 5: Transfer the seared meatballs to the oven and bake for 10-12 minutes, or until they reach an internal temperature of 165°F (74°C).
Step 6: While the meatballs bake, prepare the sauce. In the same skillet, combine heavy cream, chicken broth, Dijon mustard, thyme, salt, and pepper. Simmer for 3-4 minutes, stirring occasionally, until the sauce thickens slightly.
Step 7: Add the baked meatballs to the skillet, spooning the sauce over them. Let them simmer together for an additional 2-3 minutes.