Make the brine: Combine the salt, peppercorns, sugar, rosemary sprigs, bay leaves, 3 tablespoons each juniper berries and fennel seeds, the garlic and 2 cups water in a large saucepan. Bring to a boil, whisking occasionally, then remove from the heat. Transfer the brine to a large bowl and add 6 cups cold water; let cool completely. Submerge the pork chops in the brine, cover and refrigerate overnight.
Preheat a grill to medium. Make the roasted garlic butter: Cut off the top 1/4 inch of the garlic head and set it on a piece of foil. Drizzle with olive oil, then wrap in the foil and transfer to the grill. Cook until the garlic is softened, about 30 minutes; let cool. Meanwhile, remove the pork from the brine and pat dry with paper towels; set aside.
Squeeze the roasted garlic cloves from the skins into a food processor. Add the butter, rosemary, lemon juice and a pinch of salt and pulse until smooth. Transfer to a bowl and set aside.
Prepare the pork: Grind the remaining 1 teaspoon juniper berries and 2 tablespoons fennel seeds in a spice grinder or finely chop. Season the pork with the ground spices and salt and pepper. Lightly brush the grill grates with olive oil; transfer the pork to the grill. Cover and cook until marked, 15 minutes, rotating the pork 45 degrees halfway through to create a crosshatch pattern. Flip and repeat, cooking until an instant-read thermometer inserted into the center of the pork registers 145 degrees F.
Remove the pork chops from the grill and spread the roasted garlic butter on top. Let rest at least 10 minutes.
These are special-occasion chops-you'll need to get them from a butcher. They're super thick, so the cooking time is longer, but they're less likely to dry out.