Measure the milk and set it aside to come to room temp. Dice onion, mince garlic, and chop chicken into 1/2-inch pieces. Coarsely chop the sun-dried tomatoes and spinach.
Heat 1-1/2 tbsp 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 5 minutes until golden. Add garlic and cook for 30 seconds. Add the diced chicken, season with salt and pepper, and cook for 5–7 minutes until chicken is cooked through. Transfer the mixture to a bowl, cover with foil, and set aside.
In the same pot, melt butter over medium heat, then add flour, whisking constantly for 1 minute. Whisk in the Italian seasoning. Gradually whisk in the chicken broth, followed by the milk, stirring until smooth. Add the uncooked orzo and stir.
Turn the heat to high and bring the mixture to a simmer. Once bubbling, lower the heat to maintain a gentle simmer. Cook for 7–10 minutes, stirring occasionally and more toward the end of cooking. After 5 minutes, stir in the sun-dried tomatoes. The orzo should be almost tender and the mixture still slightly soupy after 7–10 minutes.
Add the cooked chicken, spinach, 3/4 cup Parmesan, Dijon mustard (if using), and 1 tablespoon lemon juice (if using) to the pot. Stir to melt the cheese and wilt the spinach. Taste and adjust seasoning with additional salt and pepper if needed.
The consistency should be creamy, like risotto. If it’s too thick, add a little milk to thin it out. Serve right away and top each bowl with the remaining Parmesan. You can also add a spoonful of basil pesto and fresh herbs if you like. Enjoy while it’s hot!
