Cook noodles according to package instructions.
In medium bowl, whisk together flour, salt, pepper, chicken stock and milk.
When there is about 12 minutes left for the noodles, heat oil in a large skillet over medium-high heat. When oil is hot, add sliced mushrooms and onion and sauté for 6 minutes. After 6 minutes add the garlic and sauté for 2 more minutes.
Pour your wet mixture into the skillet and bring to a simmer. Stir occasionally until thickened, about 3-4 minutes.
Reduce heat to low.
Add cheese and whisk until melted.
Stir in spinach.
Add drained noodles to the skillet and combine.