Preheat oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish.
Cook the penne pasta according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium-high heat. Season chicken pieces with salt, black pepper, and garlic powder.
Add chicken to the skillet and cook for 6-8 minutes, stirring occasionally, until cooked through and lightly browned. Remove chicken and set aside.
In the same skillet, add broccoli florets, sliced carrots, and sliced bell peppers. Sauté for 4-5 minutes until vegetables are slightly tender but still crisp. Remove from heat.
In a medium saucepan, melt butter over medium heat. Add minced garlic and cook for 1 minute until fragrant.
Whisk in flour and cook for 2 minutes to form a roux, stirring constantly.
Gradually whisk in whole milk and heavy cream, continuing to whisk until the sauce thickens, about 5-7 minutes.
Remove saucepan from heat and stir in grated Parmesan cheese, nutmeg, salt, and black pepper until smooth.
In a large mixing bowl, combine cooked pasta, cooked chicken, sautéed vegetables, and Alfredo sauce. Mix well to coat evenly.
Transfer the mixture into the prepared baking dish. Cover tightly with aluminum foil.
To freeze: Place the covered dish in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 375°F (190°C). Remove foil and cover with fresh foil. Bake covered for 45 minutes, then uncover and bake an additional 15 minutes until bubbly and golden on top.
Garnish with chopped fresh parsley before serving.