Bring the beef broth to boil, add the spaghetti. Season with salt and pepper.
In the meantime, heat the olive oil in a large skillet and cook the onion until translucent, then add the garlic, mushrooms and cilantro. Season with salt and pepper to taste.
Reduce heat to low until the mushrooms are cooked.
After the pasta is cooked, add the mushroom mix, the chili pepper (if desired) to the pot and allow broth to return to a boil. Remove from heat and serve immediately.
