Put the yolks in a metal bowl and set over a pot of gently simmering water (make sure the water does not touch the bottom of the bowl.) Season the eggs with salt, and pepper, and whisk until thick and frothy. Remove the bowl from the heat. In a separate bowl, beat the whites to stiff peaks, and fold into the yolk mixture.
Heat the remaining butter in a large frying pan for the omelette.
Cook's Note: a no-stick pan will simplify your life.
Pour the egg into the pan and cook until the bottom is set, and golden, and the top is still fluffy and soft, about 3 to 5 minutes. Scatter the spinach mixture all over the omelette, then the cheese. Cover the pan for a few minutes to soften the cheese. Uncover, and fold the omelette in half, and slide it onto a plate. Serve immediately.