braisingmeat
Question is basically as is in the title. After simmering for hours, I would expect beef or pork to look fully cooked and grey in the centre, but it'll come out with some pink in the middle. Since its internal temperature should have reached around boiling, why would the inside still look like a medium steak?
(This question isn't about food safety, more about why this happens.)
Your filling is basically a beef stew/casserole, so any cuts that suite long slow cooking are fine. They get cooked again while the pie is baked, so premium cuts will just be wasted
The lower grade and 'off cut' parts are what are most commonly used. Like rump, brisket, chuck, shank, neck etc
Using a bench top slow cooker makes cooking this all very easy. The beef needs to be fully cooked before making the pie. An eight hour slow cook is fine
You want the pieces to be a variety of sizes to fill out the pie shape, but make sure you have some at least the height of the pie filling. The larger pieces tend to be better, but that could be a matter of taste and culture
You need to get rid of plenty of fat before and after cooking. A mouth full of fat in a pie is not nice. There is already plenty of fat in the gravy and pastry, so trim off all you can
A good meat pie should have a solid layer of meat connected with gravy, not swimming. It should also not be so full as you can't eat it with your hands (for a single serve pie) without loosing half the contents
Puff/flakey pastry works best and holds everything together
Avoid overly strong flavoured gravies, you don't want to hide the beef and pastry taste
Before you attach the lid, place a slice of tasty cheese, or an extra generous serving of cracked pepper on top of the filling
A respectable pie looks like this - thanks to Ponsonby Pies (not made in Ponsonby anymore)
This deli meat you speak of is generally made of small scraps of mechanically-separated meat that are essentially "glued" together into a solid mass by enzymes that partially break down the tissue. This processed "meat brick" doesn't really have the same texture as an intact muscle tissue, which has individual muscle fibers aligned along a "grain" that makes it pleasantly chewy. Folding/layering/rolling slices of processed meat gives it more of a texture and chew, and tricks you into thinking you're eating an actual piece of animal muscle instead of meat-flavored jelly.
Best Answer
Your assumption about a temperature "near boiling" is wrong. Meat is an effective temperature insulator.
Meat in a pot always ends up having a heat gradient, with the center being colder than the outside. A thin piece of meat will end up the same temperature as the water after many hours, but a big slab will still have a few degrees Celsius difference.
I am also pretty sure that your meat doesn't look "like a medium steak". Sure, the color may not be grey, but the color needs very high temperatures to turn grey, maybe above 80 Celsius, while a steak stops being medium if it reaches 57 Celsius. There is a vast range between "medium" and "grey" where the color may still be in the pink part of the spectrum, but the texture is entirely different (and the color is also not the exact same shade, if you look closely). Combine this with the fact that simmering water is not close to boiling, it is 90-ish Celsius, and that stewing is done with meat which is high in isolating fat and connective tissues, and it is no wonder that, even after a couple of hours, the temperature has not compleltey evened out throughout a slab of meat and the middle stays a different color.