Cook pasta according to package directions, until al dente. Drain water.
In a large skillet, add butter and onion. Saute over medium heat until caramelized. Add spinach and saute for one minute, then stir in cream and gorgonzola. Cook 3-5 minutes until sauce thickens, stirring constantly. Add salt and pepper to taste.
Add pasta to sauce and toss to coat. Serve immediately and garnish with gorgonzola crumbles.