Tomahawk
Combine water, salt, sugar, bay leaf, juniper berries, and black pepper in a large container; stir until dissolved.
Submerge pork tomahawk in the brine and refrigerate in the cool room for 48 hours, keeping it below 4°C (39°F).
Remove pork from brine and pat completely dry with paper towel.
Place uncovered on a rack in the cool room for several hours or overnight to form a tacky surface.
Preheat a charcoal grill for indirect low heat (120–140°C / 250–285°F).
Place pork tomahawk on the grill away from direct flames, fat side up if possible.
Cook slowly over charcoal until internal temperature reaches 58–60°C (136–140°F).
For the last 2–3 minutes of cooking, move the tomahawk over direct heat. Brush generously with plum glaze, turning and glazing until lacquered and the internal temperature reaches 60–63°C (140–145°F).
Remove the pork from the grill and let rest for 5–10 minutes.
Carve meat off the bone and slice thickly across the grain.
Arrange on a serving platter, drizzle with olive oil, and serve with extra plum glaze.
Plum glaze
Grill halved onions, cut side down, until deeply charred, grill whole plums until blistered and grill bacon until crispy.
Bring the mirin to the boil and cook until reduced by half.
Mash the grilled plums and remove pits.
In a large pot, combine mashed plums, reduced mirin, rice wine vinegar, shiro koji, grilled onions, crispy bacon, and thyme. Simmer mixture gently over low heat for 30 minutes, stirring occasionally and avoid boiling.
Remove from heat and cool to room temperature. Sieve out thyme sprigs, onions, and bacon.
Transfer remaining mixture to a blender and blend until smooth, season.
