Finely chop the onion and mince the garlic; have the remaining ingredients measured and ready since the recipe cooks quickly.
Heat the olive oil in a large nonstick pot over medium-high heat. Add the diced onion and cook, stirring, until golden, about 4–5 minutes.
Add the minced garlic and cook, stirring, for 30 seconds.
Add the ground turkey, season with salt and pepper (about 1 teaspoon salt and 1/2 teaspoon pepper if desired), and cook, breaking up the meat, until mostly cooked through, about 5 minutes.
Stir in the frozen corn and frozen peas (no need to thaw) and cook 1–2 minutes until thawed and the meat is fully cooked; transfer the mixture to a plate and cover to keep warm.
Return the empty pot to the heat, melt the butter, whisk in the flour, and cook while stirring for 1 minute to form a roux.
Gradually whisk in the chicken stock until smooth, then add the whole milk and the uncooked orzo.
Bring to a simmer, then reduce heat so the mixture is barely bubbling at the edges. Cook uncovered, stirring occasionally and scraping the bottom, for about 7 minutes until the orzo is almost tender and the mixture is slightly soupy.
Return the cooked turkey and vegetables to the pot and stir to combine.
Add the grated Parmesan and stir until melted, then mix in the basil pesto and optional chopped parsley. Taste and adjust salt and pepper as needed.
Serve hot; the finished consistency should be creamy, similar to risotto. Add a splash of milk if you need to thin it slightly.