Can almond milk be substituted for evaporated milk in a custard pie recipe?
Almond milk is much, much thinner than evaporated milk. Do you specifically need to use almond milk, or are you just looking for a non-dairy option? I've used coconut milk for custards with some success; you might also be able to use a soy creamer (since you can get away with subbing cream for evaporated milk).
You would need to remove some of the liquid youself for this to work. I have successfully made pies by subsitituting soymilk that I had reduced to half its starting volume over low heat. Almond milk should work about the same, just needs to pour twice what your recipe calls for into a pot and simmer until it loses volume, wide surface area would speed up the process.
Using coconut milk is more efficient, as it is thick enough to work without being reduced.
As you know almond milk is not really milk at all, but almonds soaked in milk overnight say then blitzed and strained, it has the protein of milk but not any of the sugars or fats. Evaporated milk is milk with about 60% of the liquid removed and sugar added.
If you wanted to use it in a pie recipe (lemon meringue or key lime by any chance?) you would have to add additional thickener like corn-starch. I would add take 2 tbs's and mix it with a tbs of almond milk to make a paste then mix it with the remaining almond milk. I would not cook it on the hob, I would just let it cook in the pie but then perhaps that is wrong.
In addition to the extra thickener, you would have to add extra sugar but I assume this won't actually change the result of the recipe so could be adapted to your taste.
Hope this helps!