English mustard vs. "german" mustard

The German „Mittelscharfer Senf“ ist pretty wimpy compared to the average English mustard that looks deceptively similar.

You need something that packs more punch, if you can’t get proper English mustard (the Coleman’s in the other answer is occasionally available in German stores), a Dijon mustard (Maille is a commonly seen brand) will do, or a „Scharfer Senf“ (strong/sharp mustard) of a German brand.

The "easiest solution" would be to go with Gordons recipe that doesn't use mustard

Ingredients

• 2 x 400g beef fillets
• Olive oil, for frying
• 500g mixture of wild mushrooms, cleaned
• 1 thyme sprig, leaves only
• 500g puff pastry
• 8 slices of Parma ham
• 2 egg yolks, beaten with 1 tbsp water and a pinch of salt
• Sea salt and freshly ground black pepper

Based on this link and their description of "English mustard"

English Mustard: Made from both white and brown or black seeds, flour, and turmeric.
Usually bright yellow in color with an extremely hot spiciness to the tongue.

and the other answers and comments I wouldn't use Bautzner Mittelscharfer Senf as this one is really mild and not spicy at all. Dijon mustard is always a good choice (imho) but as an alternative I would suggest to use something like the quite common "Original Löwensenf extra" which brings a well-balanced mustard and vinegar taste and a good portion of heat with it.

I have found that English mustard has a pronounced turmeric flavour which brings it more in line with horseradish. German mustard is milder taste with a stronger taste of the mustard seeds. The german varieties also seem to have a bigger taste of the malt vinegar.