In a large pot of boiling salted water, cook pasta according to package instructions; drain well.
Heat olive oil in a large cast iron skillet over medium heat. Add garlic and onion, and cook, stirring frequently, until onions have become translucent, about 2-3 minutes.
Whisk in flour until lightly browned, about 1 minute.
Gradually whisk in milk, and cook, whisking constantly, until slightly thickened, about 3-4 minutes.
Stir in spinach until wilted, about 3 minutes.
Stir in Parmesan until melted, about 1 minute.
Stir in orzo; season with salt and pepper, to taste.