Steak pie filling thickener
Normally, I aim to have very little liquid in my filling before it goes in a pie, rather than thicken it a lot. It just seems to keep the pastry better &amp; saves me having to blind bake the base. I use what I've drained off to make gravy for serving.
I do thicken what liquid remains, I don't drain it all, &amp; my preferred method is with a roux.
Just use plain [all-purpose] flour, made as a roux or just mixed with water &amp; stirred in.
For a roux, I'd make that up in a separate pan. Let your pie mixture cool for a while first. Equal parts butter &amp; flour. Melt the butter, stir in the flour a few minutes, then add the meat/gravy mix. Get some of the gravy in first, until the overall mixture becomes manageable; it will start off by transforming to one lump, but if you keep adding gravy slowly &amp; keep stirring it will never come out lumpy, guaranteed. Gradually add your meat/gravy mix to it until it's all in.
If you mix flour with cold water then add to your hot mixture, you will need to be carful not to get lumps, pour in slowly, stir constantly. I'd also then let it simmer for a while, otherwise you get a bit of a raw edge to the taste. The thickness will be the same as with a roux, but the flavour isn't as good, imho.
Made this way it won't go gelatinous like cornflour would. Making it as a roux will also keep a bit of 'shine' to the sauce, more than the cold water method.