Measure the whole milk and let it sit at room temperature while you prepare the other ingredients; dice the onion, mince the garlic, cut the chicken into 1/2-inch pieces, and coarsely chop the sun-dried tomatoes and spinach.
Heat about 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, for about 5 minutes until golden; add the garlic and cook 30 seconds until fragrant.
Add the diced chicken, season with 1/2 teaspoon salt and 1/4 teaspoon pepper, and cook 5–7 minutes until cooked through and no longer pink; transfer the chicken mixture to a bowl and cover to keep warm.
In the same pot, melt 2 tablespoons butter over medium heat, whisk in 2 tablespoons flour and cook 1 minute while whisking, then whisk in 2 teaspoons Italian seasoning.
Gradually whisk in 2 cups chicken broth, then whisk in the 2¼ cups room-temperature whole milk until the sauce is smooth.
Stir in 1½ cups uncooked orzo, then bring the pot to a simmer over high heat; once bubbling, reduce heat to maintain a gentle simmer and cook, stirring occasionally, for 7–10 minutes until orzo is almost tender and mixture is slightly saucy.
After about 5 minutes of simmering, stir in the chopped sun-dried tomatoes so they soften into the sauce.
When the orzo is almost tender, return the cooked chicken to the pot along with 3 cups chopped spinach, 3/4 cup of the grated Parmesan, 1 teaspoon 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 splash of milk to reach a creamy, risotto-like consistency.
Serve immediately and top each portion with the remaining grated Parmesan; optionally add a spoonful of basil pesto or fresh herbs if desired.