In the meantime, add 2 tablespoons olive oil and onion to pan. Cook on medium heat for 3 minutes.
Add mushrooms. Cook for another 3-5 minutes or until softened. Remove mushrooms and onions from pan. Set aside.
Add pancetta to pan. Cook on medium heat for 3 minutes. Return mushrooms and onions to pancetta. Add 2 tablespoons of olive oil. Cook on low heat for an additional 3 minutes. Remove from heat.
When fettuccine is cooked, drain leaving a little water. Add sauce, green onions and cheese.