So there are all kinds of ways to vary this recipe. Let's start with the meat. Traditionally merguez is 100 percent lamb, so you can do that if you want. For venison, you can use 100 percent venison if the animal was fat (read this article on venison fat if you are nervous about using it), or 4 pounds lean venison with 1 pound pork fatback or beef fat, or 3 pounds venison with 2 pounds fatty pork shoulder. Sheep casings make this sausage, and you can sometimes get them in good butcher shops, or you can buy sheep casings online from The Sausage Maker. Yes, you can do merguez in hog casings, but it's obviously not traditional.
Get out about 20 feet of sheep casings and soak them in warm water. If you don’t trust your source, run water through them to check for punctures or weak spots.
Make sure all your equipment is cold; freeze your grinder's grinding plate and blades, and the bowl you will put the meat into for 30 minutes to an hour. Do the same for the meat and fat. When everything’s nice and cold, mix the meat and fat with the salt and all the dry spices.
Grind it all through a coarse plate; I use a 10 mm plate. Test the temperature of the mixture, and if it’s 35°F or colder, go ahead and grind it all again through a fine die, like a 4.5 mm or thereabouts. If it’s warmer, put the mix back in the freezer to chill until it hits 35°F or cooler.
Once the sausage has been ground twice, test the temperature again to make sure it’s cold. I prefer to chill the mix down to 28°F to 32°F for this next stage. Chill the mix and when it’s cold enough, take it out and add the harissa, vinegar and water. Now, mix and knead the mixture in a big bin or bowl with your hands for a solid 2 minutes — your hands will ache with cold, which is good. You want everything to almost emulsify.
Stuff the sausage into sheep casings rather loosely. It's pretty important that you have a narrow stuffing tube for your stuffer, or the sheep casings will break. I like my merguez to be about 6 to 8 inches long, but it’s your choice. To twist them into links, tie off one end of the coil you just made. Pinch off links with your two hands and roll the link between them forward a couple times. Move down the coil and repeat, only this time roll backwards a few times. Repeat until you do the whole coil. Now look at the links, which will probably have air pockets in them. Use a sterile needle or sausage pricker (set it aglow in your stovetop flame) to puncture the casing over all the air pockets. Gently compress the links together to squeeze out the air pockets and rotate the links a bit more to tighten; this takes practice.
Hang your links for at least 1 hour if your room is warm, and up to overnight if you can hang them in a place that’s 40°F or cooler. Don’t let them freeze yet. If you are not hanging overnight, let the sausages continue to dry uncovered in the fridge overnight before you seal them up and freeze. Merguez will keep a week in the fridge and a year in the freezer, if you have vacuum-sealed them.