Preheat a grill pan over medium-high heat.
Put the lamb chops in a bowl, drizzle a couple of tablespoons oil over them along with the fennel seeds, and salt and pepper and toss well to coat the meat. Let stand 10 minutes.
Slice one of the star fruit, remove the seeds, and transfer to the bowl of a food processor or blender. Add the rice vinegar and puree the fruit until fine. Pour the mixture into a small saucepan, add the brown sugar and garlic and season with salt and pepper. Bring to simmer and cook until the liquid is evaporated and the mixture is thick, about 10 minutes. Add a splash of water if the pan gets dry. It should be the consistency of drained crushed pineapple.