In a small mixing bowl, combine the yellow and brown mustard seeds, whisking them together to evenly distribute. Pour in the apple cider vinegar and your choice of beer or water, stirring gently until the seeds are fully immersed in the liquid. Cover the bowl and let it sit in a cool, dry area for about 12 hours, allowing the seeds to soak up the liquid and soften.

Once the soaking process is complete, stir in the light brown sugar, honey, and sea salt, ensuring everything is well incorporated. Transfer the mixture to a food processor. Pulse the mixture five to six times to break down the seeds slightly before running the motor continuously for 30 seconds to 1 minute, depending on your preferred mustard consistency.

Spoon the mustard into a sterilized glass jar, sealing it tightly. Refrigerate for at least 1 to 2 days before using, allowing the flavors to meld and develop into a bold, tangy mustard with the perfect balance of sweetness and spice.