Melt butter & water in a saucepan and bring to a boil.
Off heat, add flour. Turn on flame and mix until mass formed
Off heat add one egg at a time and mix well.
Add mustard, salt and cheese and taste to correct seasoning
Spoon onto a baking sheet and cook 210C for 30 min.
Remove from oven, pierce with a small knife to let steam out
Return to turned off oven for 10 min.
I usually mix gruyer cheese and parmesan cheese to make this! I recommend swiss cheese.