Heat 1 Tbsp. oil in large skillet over medium high heat; add onions and cook for 2–3 minutes, until translucent. Add mushrooms cook for 5 minutes, or until browned. Add spinach, nutmeg, salt and pepper and cook until spinach is wilted. Remove pan from heat and set aside.
Heat remaining oil over medium heat. Whisk in flour and cook, stirring 1-2 minutes, or until bubbling. Slowly whisk in milk and continue to whisk until bubbling. Remove from heat. Add cheese and mix until melted.
Add cheese to vegetable mixture and mix until well combined.
Salt & pepper to taste