Slice the biltong thin. Set aside 20 g for garnish and use 100 g for the infusion. Thin slices release flavor faster and blend more easily.
Put the stock and 100 g biltong in a pot. Warm until just about to simmer, then take off the heat and infuse at least 30 minutes, longer if you can. The biltong stays in.
Toast the coriander seeds and peppercorns in a dry pan over medium heat until fragrant and nutty, then grind fresh.
Sweat the onion and garlic in the oil with a pinch of the salt, lid on, over low heat until soft and translucent. Do not brown them.
Add the butter, Dijon, and ground spices. Cook on low about 1 minute. Stir in the flour and cook out 30 to 60 seconds.
Add the infused stock, biltong and all, a little at a time, stirring smooth between additions. Stir in the cream and Worcestershire.
Simmer gently, stirring — a lazy bubble, not a rolling boil.
Stir in the grated cheddar and blue cheese until melted, then remove from the heat. Blend smooth (vent a jug blender).
Salt at the end, once blended, tasting first. Ladle into hot bowls, top with the reserved biltong and fresh black pepper.
Biltong — Use a good fatty piece of beef biltong. It blends into the soup, so the fat is flavor, not something to trim. Slice thin so it infuses and blends well. Avoid very sweet or teriyaki-style cured meat.
The infusion (most important step) — Warming the biltong in the stock and resting it at least 30 minutes is what makes the soup taste of biltong rather than beef. It is unattended time, so start it first. Do not skip or shorten it.
Salt control — Biltong, stock, and cheese all bring salt. Use unsalted stock, only a pinch to sweat the onions, and taste at the very end before adding more.
Toasted spices — Toast the coriander seeds and peppercorns whole and grind them fresh. Much of the biltong-like aroma comes from here.
Cheese and blending — Stir the cheese in the last minute, don't boil for long, and blend smoothly. Grate from a block, not pre-grated.
Fixes — If too salty, add cream and some unsalted stock. If too thin, stir in a little cornstarch slurry or a bit more roux.
Substitutions — Gluten-free: gluten-free flour or a cornstarch slurry. Dairy-free: plant butter, oat or cashew cream, dairy-free cheddar, no blue cheese. Milder: omit the blue cheese for extra cheddar.
Storage — Freezes well in portions. Thaw in the fridge and reheat gently, loosening with warm stock. Keeps 3 days refrigerated. Garnish with fresh biltong at serving.