Whisk the water and salt until completely dissolved, then add the meaty neck bones and brine overnight.
The next day, pour off the brine and discard. Put the necks, wine, and remaining ingredients into a pot and fill to cover with fresh water.
Bring the pot to a simmer, and cook, gently skimming off fat and albumen regularly, until the meat easily removes from the bone, a few hours.
Remove the hot bones to cool, covered with cling film (this prevents them from drying out). Meanwhile, pass the stock through a fine strainer and start reducing it on medium-high heat, skimming any fat and foam, until exactly 2 cups remain. This will take a while.
Very carefully pick the meat from the bones. It’s important to only pick pieces of meat that you know will be pleasant to chew.
Work slowly-there is a lot of cartilage and funky tissue, but also pockets of amazing meat on the necks you want to save. Reserve the meat.
Freeze a small plate to do a set test (optional).
The cooking liquid should now be thickened like warm honey. Drop a teaspoon on a frozen plate, it should hold it's boundary and begin to sieze up like jam.
Taste the cooking liquid, it should taste very well seasoned-a little too strong, salty and rich. If it tastes too salty for you at first don't worry, it's going to be dispersed in a bunch of meat, and you want it strong since it's served cold.
Meanwhile, remove the meat from the fridge, and inspect it, if it looks like a lot of fat has congealed on it overnight, put the meat in a colander and gently pour drops of boiling water from a teapot or similar on problem spots to remove excess fat. If the meat doesn’t have too much fat on it, great, skip it.
Meanwhile, coarsley chop the meat and mix well with the reduced stock, then pack the mixture into the terrine mold lined generously with cling film, pressing cling film onto the surface to hold everything down. Chill until completely set, 3-4 hours or preferably overnight.
To serve, carefully unmold the terrine, remove the cling film, inspect the bottom for any unwanted particles that may have risen to the top and trim, then use a long sharp knife to cut ½ inch slices. Don't forget the horseradis
Notes
Black Trumpet Mushroom Inlay(optional)
25 grams dried black trumpet mushrooms (two generous handfuls, or ¾ cup lightly crushed dried mushrooms)
2 tablespoons flavorless cooking oil
1 teaspoon chopped fresh thyme, or more to taste Kosher salt
Rehydrate the trumpets for 15-20 minutes in water just to cover. Strain the water and add to to beef neck cooking liquid.
Meanwhile, coarsely chop the trumpets and sweat in the oil, seasoning with the salt and thyme. Don't brown them, just sweat and stir to warm them and distribute the seasonings.
From here the trumpet inlay can be made ahead of time. When packing the terrine into the mold, lay down half of the beef, put the trumpets in the middle, then put the rest of the beef on top.