Heat olive oil in a pan.
Add onion and cook until softened.
Add mushrooms and cook until browned.
Add garlic and cook for 1 minute.
Add orzo and toast for 1-2 minutes.
Pour in chicken stock and milk.
Stir in Dijon mustard.
Bring to a simmer and cook for 10-15 minutes, stirring occasionally, until the orzo is cooked and the liquid has reduced.
Stir in baby spinach and Parmesan until the spinach has wilted and the cheese has melted.
Season with salt and pepper to taste.
