Cook pasta in salted water, following packet's instruction.
Heat some oil in a deep frying pan or wok.
Add the sliced mushrooms to the oil, saute until brown.
Add the spinach and toss with the mushrooms over heat, until wilted.
Turn down heat on pan to low, add peanut butter and ketchup, and stir in to cover the mushrooms and spinach.
When pasta is ready, drain and then mix into the mushroom/spinach mix.
Optionally, top with parmesan or other hard cheese, and/or pine nuts.