Turn on stove burner to low heat.
Coat tamagoyaki pan with butter. Holding the pan's handle, turn the fry pan back and forth and side to side to ensure all the corners get covered.
With a fork or chopsticks, put a small amount of egg on the surface of the pan. If the egg sizzles, the pan is hot enough.
Pour enough egg in pan to make a thin layer. Slightly tilt the pan and forth and side to side so the egg coats the entire bottom of the pan.
Once the first layer of egg is half cooked, add fillings on top of one another one-third of the layer of egg (in this order: smoked salmon, cheese, spinach). See photo below.
With a spatula, roll up the tamagoyaki towards you to carefully wrap fillings inside. Keep rolling until you reach the end of the pan closest to you.
Once completely rolled up, nudge the roll to the far side of the pan, away from you.
Repeat the process from step 2- recoat the pan with butter, add egg mixture, and roll. Continue this process until you run out of egg. Tip- When adding the second and third layers of egg, gently list the tamagoyaki roll with a spatula so the raw egg can slide underneath the roll and coat the entire pan.