Preheat the oven to 375°F. Grease a 9x13-inch baking dish.
Cook the penne pasta in a large pot of salted water according to the package instructions until al dente. Drain the pasta and set aside.
Meanwhile, in a large skillet, melt the butter over medium heat.
Add the diced onion and cook until softened and translucent, about 5 minutes.
Add the minced garlic to the skillet and cook for an additional 30 seconds, stirring the entire time.
Sprinkle the flour over the onion and garlic mixture, stirring to combine. Cook for 1-2 minutes to remove the raw flour taste.
Gradually whisk in the chicken broth and heavy cream, stirring constantly to prevent lumps from forming.
Add the Dijon mustard, thyme, salt, and pepper. Bring the sauce to a simmer and cook, stirring frequently, until thickened, about 5 minutes.
Stir together the sauce, cooked turkey, frozen peas, cooked penne pasta, and 1 cup of the shredded cheddar cheese in the prepared baking dish, spreading it out evenly.
Sprinkle the remaining 1 cup of shredded cheddar cheese evenly over the top of the casserole.
Bake the casserole for 25-30 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Remove the casserole from the oven and let it cool for 5-10 minutes before serving.
Notes
Penne pasta: Substitute with any short pasta like rigatoni or ziti.
Turkey: Substitute the chopped turkey with shredded turkey, or you can use cooked chicken instead.
Cream: For a lighter option, use evaporated milk in place of the cream.
Mustard: Or use a similar mustard, like stone ground mustard.
Thyme: Dried rosemary also works well in this recipe.
Cheddar cheese: Use any melty cheese a cheddar cheese blend.