Prepare the egg and toast the bread.. Add the oil or butter to a small 6 to 8-inch (15 to 20 cm) nonstick skillet and warm it over medium-low heat. Crack in the egg and season with a very small pinch of salt. Cook for about 1 minute, until the white just begins to set. Add 1 teaspoon of water (5 ml) to the edge of the pan, not on the yolk, and cover with the lid right away. Cook for 1 to 2 minutes, until the steam sets the white while the yolk stays soft. Toast the sourdough until golden and crisp at the edges, then set it on your plate.
Spread the cream cheese. Spread the cream cheese over the toast while it is still hot, because it softens and spreads easily.
Layer the salmon. Drape the smoked salmon in loose folds over the cream cheese. Do not flatten it, since the folds look better and taste better.
Add the spinach. Pile the dry spinach on top.
Add the egg. Place the egg on top of the stack and add the arugula if you are using it.
Serve. Decorate optionally with a cherry tomato cut into thin rounds, then arrange the tomato slices on top. Serve right away while the yolk is still runny.