As an American, I only put mustard on beef when it’s part of a sandwich. But some British novels I’ve read suggest Brits put mustard on roast beef on the plate, and a “mustard-pot” is a typical item on the table. Is this true? Is it good that way? Does it have to be “English” mustard?
Yes, it’s true, though horseradish is more popular these days. And yes, it must be English mustard, which is intensely hot. A small spoonful of mustard is put on the side of the plate, and only the smallest smear of mustard is added to each forkful of beef before consuming.
The bespoke mustard pot, that had a lid and a teensy little spoon, does exist but is dying out, sadly. My late grandfather had one - he’d decant a jar of Coleman’s into the pot, but years before that, mustard would have been mixed up from a dry powder, directly in the pot.
Here’s a fine example of a contemporary mustard pot.
Also, I failed to answer your question “Is it good that way?”. It’s all a matter of taste, of course, but personally speaking I absolutely love it. It adds a burning top-note to the wide savoury taste of the beef.
I have several antique mustard pots. Every now and then I’ll fill one up with freshly-mixed mustard and enjoy the sinus-clearing properties while dining on beef. Quite tasty.
Colman’s - the one good thing to come out of Norwich 
English mustard also will run a little into the gravy and the juice from the (hopefully pink) beef, which transfers a little of the flavour to anything else on the plate.
Mmmmm.
Mustard and gravy! And they say British cuisine is bland!
Mmmm…mustard on roastbeef. Though still not as good as horseradish. Which is also not bland.
Whereas I’ve seen Americans put mayo on roastbeef. And most other things. And even our mustard is bland. And let’s not speak of our cheese.
Yet again I must stick up for some American cheese. Sugarbush Farm in Vermont does the best cheddar I have ever tasted.
We do this too, although whatever type of mustard is available is fine.
Is this powder still available? In the U.S.? Is it the same thing as the “Ground Mustard” you would find in the spice aisle of a supermarket? Do you mix it with water, vinegar, or what?
Sure is. Given that the amounts are in Imperial (“English”) measures - ounces - strikes me that this might be a US site. You mix it with water. It’s also great to put in stock. Not sure if it’s the same as “ground mustard” - I would imagine that’s just ground mustard seed.
Yes. It’s in my cupboard now. I prefer it to the jar version, because you can make it slightly thicker.
No idea.
With water. If it’s any help, it’s Colman’s, and lists “Ingredients: mustard flour” 
[QUOTE=GorillaMan]
Colman’s - the one good thing to come out of Norwich ;)/QUOTE]
It’s been said that Colemans made their fortune because of the fact that people always mixed up too much mustard. Most of it was left either on the side of the plate or congealing in the pot.
The same Coleman’s will allow you to duplicate that searing Chinese mustard for your takeout eggrolls. The stuff in the plactic packs is just water compared to what they give you at your table. Making your own allows you to burn the hairs right out of your nostrils.
Order it from Penzey’s. Fresher and cheaper than what your supermarket has.
Come on, chaps. Colman’s. No E. If they’re going to search for it, give them a chance of hitting it with the right spelling!
I got it right second time.
It has to be Colmans, I agree. Forget that bulk stuff.
Penzey’s is better. I’ve got both in the cupboard.
I prefer my beef unadulterated, but then spice doesn’t do me. 
Ah well I was speaking of the cheese known as American. Which I’m not sure is cheese. Or food. And this is what gets slapped on a lot of American food. When they’re out of mayo. And didn’t we invent Cheez-Wiz? But point taken, I’ve had plenty of good American cheddar.
I will still come out as against yellow mustard.