In a large pot of boiling, salted water, cook spaghetti according to package directions until al dente. Drain; set aside.
Toast pine nuts in a large skillet over medium heat, about 3 minutes. Remove from skillet and let cool.
In same skillet, melt butter. Add onion and garlic; cook for 1 minute. Add mushrooms; cook until soft, about 5 minutes.
Add broth; bring to a boil. Add spinach; stir until wilted. Add spaghetti and toss. Add tomato and provolone; toss. Season with salt and pepper. Top with pine nuts, if desired.