Brown the ground turkey: Heat the olive oil in a large skillet over medium-high heat and add the ground turkey. Break it into small crumbles with a spatula and cook until no pink remains, about 6 minutes.

Soften onion and celery: Add the diced onion and celery to the skillet with the turkey. Cook, stirring occasionally, until the vegetables turn soft and translucent, about 4 minutes.

Make the roux: Push the turkey and vegetables to the side of the skillet, melt the butter in the cleared space, and stir in the flour. Cook for about 1 minute, stirring constantly, until it smells nutty rather than raw.

Whisk in the broth: Slowly pour in the chicken broth while whisking continuously to prevent lumps. Scrape up any browned bits stuck to the bottom of the pan as you stir.

Add milk and pasta: Stir in the milk and uncooked pasta shells, then bring the mixture to a gentle simmer. Cover the skillet and reduce the heat to medium-low.

Simmer until tender: Cook, stirring occasionally so the pasta doesn't stick to the bottom, until the pasta is tender and the sauce has thickened, about 12 to 15 minutes.

Stir in vegetables: Add the frozen peas and carrots during the last 3 minutes of simmering so they heat through without overcooking.

Season to taste: Stir in the poultry seasoning, salt, and black pepper. Taste and adjust seasoning as needed.

Rest before serving: Remove the skillet from the heat and let it rest for 5 minutes; the sauce will thicken slightly more as it sits. Garnish with chopped parsley and serve warm.