You can just use whole egg. You can also add liquid, whether water, milk, or cream.
My preference especially given it’s a savory pie is to add a pinch of salt rather than adding liquid. The salt denatures and loosens the proteins, making it more fluid. You only need a tiny pinch and this is suitable for sweet and savory doughs.
Even Cook’s Illustrated recommends this.
“We did find, however, that in addition to flavoring the wash, salt helps denature the proteins in the egg, making it more fluid and therefore much easier to brush evenly and gently over delicate doughs”
As stated, you also need to whisk it well. I often like straining egg wash.
If you want to see the effect of different washes/glazes, you can watch this video.
You don’t need to be nervous when brushing a wash on a crust.
