Mince the onion and slice the cremini mushrooms. Beat the five eggs well in a bowl and season generously with pepper and salt. Have the fresh spinach and pine nuts within reach.
Heat the olive oil in a large skillet and sauté the onion until translucent. Add the sliced mushrooms and cook until golden brown. Add the fresh spinach in batches. Wait each time for the previous portion to wilt before adding new leaves, and cook until all the liquid released from the vegetables has evaporated from the pan.
Pour the beaten eggs evenly over the vegetable mixture in the pan. Immediately sprinkle the two tablespoons of pine nuts over the egg mixture. Turn the heat to the lowest setting, cover the pan, and let the frittata set gently and cook through for about eight minutes.
Remove the lid from the pan and check that the top of the egg is completely set. Remove the pan from the heat and let the dish rest for a moment. Carefully slide the frittata onto a cutting board, cut it into wedges, and serve immediately.