Try this fluffy omelette with fresh or frozen spinach.
Wash the spinach and cut off the stems. Spin the excess water out. Drop the spinach into a large pot on the stove to wilt, about 3 minutes. (There will be enough water still clinging to the leaves to steam it.) When the spinach is cooked, drain and plunge into an ice-bath to seize the colour. Squeeze dry in a tea towel. Chop.
Heat half the butter in a frying pan and gently sauté the shallot. Add the spinach, and season with salt, pepper, and nutmeg. When hot, turn off the heat and set aside.
Put the yolks in a metal bowl and set over a pot of gently simmering water. Season with salt and pepper and whisk until thick and frothy. Remove from the heat. Beat the whites to stiff peaks and fold into the yolk mixture.
Heat the remaining butter in a large frying pan for the omelette (no-stick will simplify your life). Pour the egg into it and cook until the bottom is set and golden and the top still fluffy and soft, three to five minutes. Scatter the spinach mixture all over the omelette, then the cheese. Cover for a few minutes to soften the cheese. Uncover, fold the omelette in half and slide it onto a plate. Serve.