In a medium bowl, combine yellow mustard seeds, brown mustard seeds, apple cider vinegar, and enough water to cover the seeds completely. Allow this mixture to soak for 12 to 24 hours at room temperature.

While the mustard seeds are soaking, mix paprika, cayenne pepper, turmeric, and salt in a small bowl until well combined.

Once soaked, drain excess liquid, then transfer the seeds to a blender. Add the spice blend and chosen sweetener. Blend on medium speed until slightly coarse, adjusting consistency with extra water or vinegar if necessary.

Transfer the blended mixture to a medium saucepan over medium-low heat. Stir constantly for about 10 minutes, allowing flavors to meld and mustard to thicken.

Remove from heat, allow to cool for a few minutes, then blend again to achieve desired smoothness.

Let cool to room temperature before transferring to a clean jar. Refrigerate for at least 24 hours before use.