01 - Preheat oven to 400°F. Grease a medium baking dish (about 2 quarts) with butter or nonstick spray.
02 - Heat olive oil in a large skillet over medium heat. Add chicken pieces and season with salt and pepper. Sauté until lightly browned but not fully cooked through, about 4–5 minutes. Transfer chicken to a plate and set aside.
03 - In the same skillet, melt butter over medium heat. Add diced onion and sauté for 2 minutes until translucent. Stir in minced garlic and cook for 1 minute until fragrant.
04 - Sprinkle flour into the skillet and cook, stirring constantly, for 1 minute to form a roux. Gradually whisk in chicken broth, followed by heavy cream and lemon juice. Continue cooking and stirring until the sauce thickens, about 3 minutes.
05 - Remove skillet from heat. Stir in dried thyme, ground nutmeg, half of the Parmesan cheese, and half of the mozzarella cheese until fully melted and the sauce is smooth.
06 - Fold the seared chicken pieces and asparagus into the cream sauce, tossing gently until everything is evenly coated.
07 - Transfer the mixture into the prepared baking dish. Sprinkle the remaining mozzarella and Parmesan evenly over the top. Add a layer of breadcrumbs if desired for extra crunch.
08 - Bake in the preheated oven for 25–30 minutes, or until the chicken is cooked through, the asparagus is tender, and the top is golden brown and bubbling.
09 - Remove from the oven and let the casserole rest for 5 minutes before serving to allow the sauce to set.