Dice the red onion and soak in a bowl of cold water for about 15 minutes. After soaking, strain off water and pat the onions dry.
Cut off the tops of the jalapeños and remove the seeds and ribs. Finely dice the flesh into evenly sized cubes. Chop up as much of the seeds and ribs as you would like for extra kick. For a mild relish with little to no heat, leave the ribs out.
Sort through the cranberries and remove any that are soft or mushy.
In a food processor or blender, pulse the cranberries in short bursts. You want them finely chopped but not mushy.
Add the pulsed cranberries to a large bowl. Add in the sugar, salt, mustard seed, coriander, and ginger. Stir to combine and fully incorporate with the chopped cranberries.
Add in the onion, jalapeño, and orange zest. Stir to combine.
Pour in the vinegar and orange juice. Stir to combine.
Cover and allow to rest in the fridge (or transfer to a jar with a tight lid) for at least 24 hours.
Once flavors have come together, taste and adjust salt and sugar as needed.