1. cook linguine according to package directions in a skillet, heat butter over medium heat.
2. add onions and saute until softened.
3. add mushrooms, saute about 5 minutes.
4. stir in flour with a whisk, and stir in cream, whisking constantly.
5. raise heat to high and bring to a boil, then lower to simmer and cook until sauce thickens, about 3 minutes.
6. remove from heat and stir in cheese, salt, pepper.
7. cmbine the linguine, spinach, and sauce in a large bowl, tossing to coat.