Leeks must be carefully washed, as dirt tends to get between their layers. I often chop the pieces first, then wash them in a colander, and dry them well before sautéing.
The amount of salt to be added depends on how salty the feta is. While you shouldn’t try the raw egg mixture (unless you use pasteurized eggs), you can taste the feta separately. Personally, I prefer to undersalt and let each guest add salt if they prefer more.
If you make the mold in advance and refrigerate it unsliced or as slices on a platter, leave enough time for the slices to come to room temperature (or close to it) before serving.