Add the ground spices in a bowl, together with the paprika, cayenne, thyme, and mustard powder.
In a large tray or bowl, add the duck and chicken, then sprinkle with the spice mix, salt, milk powder, and garlic.
Toss until evenly coated. Spread out the meat mixture in a single layer for faster cooling.
Place the container uncovered in the freezer, until the mixture is 2 degrees C or colder, around 1 hour.
While the meat is resting in the freezer, sautee the onions. In a small skillet over medium heat, heat the fat or oil.
Add the onions and cook, stirring, until soft and slightly browned on the edges, about 5 minutes. Remove from heat and let cool.
When the meat mixture is sufficiently cold, add it to your food processor in batches, with a portion of the onions.
Pulse the mixture in 1-second bursts, about 20 times. Repeat for the remaining meat mixture and onions.
You can also hand chop about 1/4 of the mixture for better texture. Here's a good video on how to do it.
Return the processed meat mixture to the tray used for chilling the meat. Place in the freezer, until the mixture is very cold (around 0 degrees), another 30 minutes.
Add the juice or vinegar mixture.
Mix the liquid into the meat, by squishing the ingredients using very clean or gloved hands. If using an electric mixer, keep the paddle and bowl chilled in the fridge until ready to use, then mix for about 1 minute 30 seconds.
Do a taste test of your sausage mixture. Adjust seasoning to taste.
Tie off one end of your prepared sausage casing, and stuff with the meat mixture.
Twist into links, and tie off the other end.
Hang the sausages to dry at room temperature, at least 1 hour, up to a maximum of 2 hours.
While hanging your sausages to dry, prick any visible air pockets to release any trapped air, using a large sterilized needle.
At this point you can store the sausages in the freezer until ready to use, and use as desired. If you're planning to smoke these sausages, proceed to the next step.
Smoke in a stovestop smoker, using 3 tablespoons of hickory or apple woodchips, until the internal temperature registers around 68 degrees C on an instant-read thermometer, 20 to 25 minutes.
Remove from heat and let completely cool before storing.