Melt butter in a medium saucepan. Add flour, cook 1 minute, stirring constantly. Gradually add milk stirring constantly - bring to a boil, then reduce heat to a simmer, stirring constantly, for 3 minutes or until sauce thickens slightly.
Remove from heat and stir in cheese, salt & pepper.
Drain pasta, reserve 1/2 cup of pasta water if needed for sauce. Combine sauce, pasta & spinach to combine. Serve it up!