Measure 2-1/4 cups whole milk and let it come to room temperature while you prep the rest.
Dice the onion, mince the garlic, coarsely chop the sun-dried tomatoes and spinach, and cut the chicken into 1/2-inch pieces.
Heat 1-1/2 tablespoons of the oil from the sun-dried tomato jar in a large nonstick pot over medium-high heat.
Add the diced onion and cook, stirring occasionally, about 5 minutes until golden and softened.
Add the minced 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.
Transfer the cooked chicken mixture to a bowl, cover with foil, and set aside.
In the same pot, melt 2 tablespoons butter over medium heat. Whisk in 2 tablespoons flour and cook 1 minute, then whisk in 2 teaspoons Italian seasoning.
Gradually whisk in 2 cups chicken broth, then the room-temperature milk, stirring until smooth.
Add 1-1/2 cups uncooked orzo to the pot and stir to combine.
Increase heat to high and bring to a simmer, then reduce to maintain a gentle simmer and cook 7–10 minutes, stirring occasionally; after 5 minutes stir in the chopped sun-dried tomatoes.
When the orzo is almost tender and the mixture is slightly soupy (within the 7–10 minute window), return the cooked chicken to the pot along with 3/4 cup of the Parmesan, the chopped spinach, and the Dijon mustard 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 serving with the remaining grated Parmesan.