Step 1

In a wide pan, heat olive oil over medium heat.

Step 2

Add mushrooms to oil and sauté, stirring occasionally, until mushrooms have reduced in size and begun to brown, about 6-8 minutes.

Step 3

Add garlic, onion, salt, and pepper to pan and stir to combine. Cook, stirring occasionally, for 4-5 minutes until onion is soft and translucent.

Step 4

Add flour to pan and stir to form a roux. If the pan looks especially dry, add an extra splash of olive oil and stir until no dry flour remains. Let cook for 30 seconds or so to cook out any raw flour taste.

Step 5

Add milk to sauce a little at a time, stirring constantly to break up any lumps of flour. Bring sauce to a simmer and cook, stirring occasionally, for 5-8 minutes or until sauce has thickened slightly.

Step 6

Turn off the heat and stir in the parmesan cheese. Taste and add salt and pepper if necessary. Serve immediately.