Melt 2 tablespoons of the butter in a small pan. Whisk in the flour and let it cook a bit (you are making a roux here), but not brown.
Whisk in hot (not boiling) water a bit at a time, then cook the sauce over medium-low heat until it thickens, about 5 minutes, and then whisk in lemon juice and salt.
Just before serving, add the remaining butter in small pieces, and whisk it in.
Notes
You may add a pinch of pepper (preferably white) if you wish.