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 spinach
Heat oil 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 stock, 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. 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), 1 tbsp lemon zest & 1/4 cup juice 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 1/4 cup Parmesan. Enjoy while it’s hot!
Notes
Note 1: Whole milk is essential for a thick and creamy texture. Lower-fat milk won’t achieve the same result.Note 2: Orzo is a small pasta, not a grain (don't use rice!). Be sure to use 1-1/2 cups and not the whole box. Quality of Orzo can differ quite a bit brand to brand; I like DeLallo or BarillaNote 3: Grab a block of Parmesan and grate it on the small holes of a grater. Avoid using pre-grated Parmesan from a can as it’s too salty and won’t melt as smoothly.Storage: This dish is best fresh, but if you need to store it, let it cool and keep it in an airtight container in the fridge for up to 3 days. When reheating, add a little milk or broth to make it creamy again.