Heat the olive oil in a large skillet over medium heat. Add the sliced chicken sausage and cook for 3-5 minutes, stirring occasionally, until lightly browned.
Add the diced onion and cook for 2-3 minutes, until softened. Stir in the garlic and cook for 30 seconds, just until fragrant.
Add the dry orzo to the skillet and stir for about 1 minute to lightly toast and coat it in the oil and sausage drippings.
Pour in the chicken broth, add the Italian seasoning, salt, and pepper. Bring to a gentle boil, then reduce to a simmer. Cook, stirring often, for about 10 minutes, until the orzo is tender and the mixture looks saucy. During the last few minutes of cooking, add the spinach and tomatoes, stirring them in until the spinach has wilted and the tomatoes are warm.
Lower the heat and stir in the milk. Add the Parmesan a little at a time, stirring until melted and smooth.
Remove the pan from the heat. Squeeze the lemon juice over the top and taste. Add more lemon juice, salt, or pepper as needed.
Serve warm with extra Parmesan, cracked black pepper, and freshly chopped basil on top.
