Prepare the Chicken: Pat the chicken breasts dry with paper towels. In a small bowl, combine salt, pepper, garlic powder, and onion powder. Sprinkle the spice mixture evenly over both sides of the chicken.

Sear the Chicken: Heat olive oil in a large skillet over medium-high heat. Place the chicken breasts in the hot skillet (cook in batches if needed). Sear for 4-5 minutes per side, until golden brown and cooked through (internal temperature of 165°F/74°C). Remove from skillet and set aside on a plate, covered with foil to keep warm.

Make the Creamy Sauce: In the same skillet, melt butter over medium heat. Whisk in flour and cook for 1-2 minutes, stirring constantly, to create a roux. Slowly pour in chicken broth, whisking continuously to prevent lumps. Add heavy cream and milk, stirring well.

Flavor the Sauce: If using, add sherry. Stir in dried thyme, dried rosemary, and nutmeg. Bring the sauce to a simmer, then reduce heat to low. Let it simmer for 5-7 minutes, stirring occasionally, until the sauce has thickened slightly. Stir in Parmesan cheese. Taste and season with salt and pepper to taste.

Cook the Rice: In a medium saucepan, combine rice, chicken broth (or water), butter, and salt. Bring to a boil over high heat. Reduce heat to low, cover, and simmer for 18-20 minutes, or until all liquid is absorbed and rice is tender. Fluff with a fork.

Assemble the Dish: If using mushrooms and onion, sauté them in the skillet after removing the chicken and before making the sauce. If using frozen peas, stir them into the creamy sauce during the last few minutes of simmering.

Serve: Place a generous serving of rice on each plate. Top with a chicken breast. Spoon the creamy sauce generously over the chicken and rice. Garnish with chopped fresh parsley, if desired. Serve immediately.