My steak pie filling is missing something.

[Fluffycloudland77]

It's steak, salt, pepper, onion and a tb spoon ketchup.

It doesn't taste quite right somehow. I have allergies so using a stock cube is out.

Should I add some chopped celery or mushroom?.

Soy sauce?

Chilli?

Suet pastry is the only type I can make, I buy the rest.
I have a gorgeous pink pudding bowl.

Red wine, soya sauce and some well reduced beef stock

You can simulate the essence of mushrooms by putting small (less than a teaspoon) of Asian Fish Sauce (which is fermented fish) and adds umami to meat dishes. I add it to all my stews. It's a great substitute for MSG (which I too am allergic to).

Sherry Vinegar is good too.

What about a dark Ale?

Garlic and a little chilli

White pepper
Underused imo and peps up many dishes

Half a can of Guinness

Guinness/stout/porter of some kind and mushrooms (larger ones with more flavour)

Celery salt (if you can have it)

Kidneys and smoked paprika

Why don't you make your own stock and freeze it?
Beer, but not whatever, is also good in steak puddings, but it would have to be a "pure" one, like a German pils or Altbier.