3 tablespoons yellow mustard seeds
1 tablespoon brown mustard seeds
3 teaspoons minced fresh thyme, divided
2 teaspoons minced fresh rosemary
1/3 cup apple cider vinegar
1 teaspoon light brown sugar
3/4 teaspoon salt
Stir together mustard seeds, 2 tsp. thyme, the rosemary, 1/3 cup water, and the vinegar in a bowl until seeds are submerged. Let sit at room temperature, covered, 2 to 3 days.
Put mustard mixture in a blender along with brown sugar and salt and blend until mixture is thick but still coarse-textured. Stir in remaining 1 tsp. thyme.
Make ahead: 2 weeks, covered and chilled.
Recommended for: Vinaigrettes. Also good rubbed under chicken skin before roasting and stirred into beef, lamb, or pork stew before the end of cooking.