Pour a small amount of spring water out of the bottle. Add sea salt, put the cap back on, and shake until dissolved; set brine aside.
Inspect a gallon-sized jar for cracks. Sterilize in simmering water.
Cut off the blossom ends (opposite the stem ends) of cucumbers. Cut into thick rounds or spears, or leave whole. Slice each piece of horseradish root into 4 pieces. Mix peppercorns, mustard seeds, and pepper flakes together in a small bowl.
Lay 2 heads of dill in the bottom of the sterilized jar; top with 1/3 of the onion rings, 1/3 of the garlic cloves, and 1/3 of the cucumbers, then pack down tightly. Sprinkle in 1/3 of the peppercorn mixture and 1/3 of the horseradish pieces. Repeat these layers twice more; jar will be fully packed except for top 1 to 2 inches. Fold up mustard greens and pack them tightly at the top. Pour in brine until completely full and covering mustard greens. Make sure there are no bubbles stuck inside the jar. Reserve excess brine.
Place a small plate at the top of the jar to keep the contents weighed down. Cover with a towel and wrap jar top with a rubber band or twine to keep in place. Store in a cool location (60 to 80 degrees F, 16 to 27 degrees C) for 1 week; check brine level daily, adding more as needed to keep everything submerged.
After 1 week, sample pickles for flavor. Cover and continue to ferment to desired flavor, up to 3 more weeks.
Once pickles are done to your liking, divide into smaller, sterilized Mason jars and seal tightly. Store in the refrigerator. If taste is too strong, pour out part of the brine and add spring water.