Heat the olive oil in a large non-stick pan over high heat.
Add the mushrooms and cook until softened (around 3 minutes). Remove from the pan and set aside.
Lower the heat to medium and melt the butter. Add garlic and onion, sauté until onion is translucent (about 3 minutes).
Add the flour, stir, and cook for 1 minute.
Pour in the milk and chicken broth, whisking to combine with the flour mixture.
Increase heat to medium-high. Add the orzo, salt, and pepper. Stir, cover, and simmer for 8-10 minutes until orzo is cooked and saucy.
Remove pan from heat. Stir in the parmesan, spinach, and cooked mushrooms. Adjust seasoning if needed.