Measure the milk and leave it at room temperature. Dice the onion, mince the garlic, cut the chicken into 1/2-inch pieces, and coarsely chop the sun-dried tomatoes and spinach.
Heat 1½ tablespoons of oil from the sun-dried tomato jar in a large nonstick pot over medium-high heat. Add the diced onion and cook, stirring occasionally, until golden, about 5 minutes.
Add the minced garlic and cook 30 seconds, then add the diced chicken. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring occasionally, until the chicken is cooked through, about 5–7 minutes.
Transfer the cooked chicken mixture to a bowl, cover with foil, and set aside.
In the same pot, melt the butter over medium heat. Whisk in the flour and cook, whisking constantly, for 1 minute to form a roux. Stir in the Italian seasoning.
Gradually whisk in the chicken broth, then whisk in the room-temperature milk until the sauce is smooth. Stir in the uncooked orzo.
Bring the mixture to a simmer over high heat, then lower to maintain a gentle simmer. Cook, stirring occasionally and more often toward the end, for 7–10 minutes. After about 5 minutes of simmering, stir in the chopped sun-dried tomatoes.
When the orzo is almost tender and the mixture is slightly saucy, stir in the reserved cooked chicken, chopped spinach, 3/4 cup of the grated Parmesan, Dijon mustard (if using), and 1 tablespoon lemon juice (if using). Stir until the cheese melts and the spinach wilts.
Taste and adjust seasoning with additional salt and pepper if needed. If the mixture is too thick, thin with a little milk. Serve immediately topped with the remaining Parmesan.
