- Preheat the oven to 190 °C (375 °F).
- Line a 33 x 23-cm (13 x 9-inch) baking Pyrex dish with parchment paper and butter.
- In a saucepan, brown the bacon. Remove some fat, if necessary.
- Add the onion and cook until tender. Add the spinach and garlic. Cook until the spinach has wilted. Season with salt and pepper. Set aside in a bowl.
- Clean the saucepan and melt the butter. Add the flour and cook for 1 minute while stirring. Add the milk and stir until it thickens.
- Add 250 ml (1 cup) of cheddar cheese and blend well. Remove from the heat and add the egg yolks. Season with salt, pepper and nutmeg.
- In a bowl, beat the egg whites until soft peaks form. Fold into the béchamel sauce. Spread this mixture in the baking dish. Smooth the top with a spatula.
- Bake at the bottom of the oven for about 15 minutes.
- Right out of the oven, flip the omelet on a clean cloth and remove the parchment paper. Spread the remaining cheese and the well-drained spinach on the omelet.
- Roll firmly to get a 23-cm (9-inch) long roll. Wrap in foil and reheat in the oven for 10 to 15 minutes. Slice to serve.