• Combine the yellow and brown mustard seeds, apple cider vinegar, and your chosen liquid in a clean glass bowl or jar. Stir well and cover.
• Let soak for 48 hours in a cool place at room temperature. If your kitchen is warm or hot, refrigerate the mixture during the soak. The seeds should become plump and softened; some liquid may remain. If you need more time before blending, keep the mixture covered in the refrigerator.
• Transfer the seeds and all remaining soaking liquid to a food processor. Add the salt, honey, and 1 tablespoon brown sugar.
• Pulse to begin breaking up the seeds, then process until the mixture is thick and spreadable, with some seeds remaining intact. Stop and scrape down the bowl as needed. If the mixture is too thick for the blades to move it, add your chosen liquid 1 teaspoon at a time, processing after each addition.
• Spoon the mustard into a clean glass jar, cover tightly, and refrigerate for 1–2 days before serving. Freshly blended mustard can taste bitter or harsh; this resting time allows the flavor to mellow.
• Stir and taste the mustard after it has rested. For a sweeter mustard, stir in additional brown sugar, 1 tablespoon at a time, using no more than 3 tablespoons total. Refrigerate after adjustin

Notes

• The liquid you choose contributes to the finished flavor. Use drinking Champagne, not Champagne vinegar.
• The mustard will thicken as it rests. If needed, stir in a small amount of water to restore a spreadable consistency.

Storage

• Keep tightly covered in the refrigerator and use a clean utensil when serving. This recipe does not have a verified refrigerated shelf life and is not intended for canning or room-temperature storage.