I’m very happy you mentioned this!

I enjoy mustard with a COLD roast beef sandwich. When I’ve had HOT roast beef, it’s usually with a different starch (potato / rice) or with a separate gluten roll albeit not as a sandwich. And when HOT, not with mustard.
this is a good Seinfeld question! Well maybe he’s already covered it.

I’ve sometimes had a steak sandwich but NOT with mustard. Usually it’s a warm sandwich.

here’s an idea - sometimes people eat a cold turkey sandwich with mayonaise - I’ve even seen cold roast beef sandwiches with mayonaise. Yet we also don’t see mayo with a hot turkey or roast beef entree.

one thing that goes with almost any sauce in HOT condition - a burger! Yet when one eats a “hamburger steak,” usually they don’t have it with condiments other than ketchup.

We never have had such deep / degenerate / depressing / useless conversations on ft before March 2020. Wow. Go us! We are the champions!!