Start by cleaning your mackerel. Remove all scales and guts, then rinse well under cold water. Pat the fish dry with paper towels and set aside.
Mix your brine in a large bowl. Combine 3 cups water, ¾ cup sea salt, ¼ cup brown sugar, minced garlic (one whole bulb), 1 tablespoon black pepper, and 1 tablespoon vinegar. Stir until salt and sugar dissolve completely.
Place the cleaned fish in the brine mixture. Make sure all pieces are fully submerged. Cover the bowl and refrigerate for 6 hours at 4°C/39°F.
While waiting, soak your wood chips in water until they're fully saturated.
After 6 hours, remove fish from brine and pat thoroughly dry with paper towels. Let the fish air dry on a rack for one hour at room temperature.
Place your soaked wood chips in a roasting pan and put it on your barbecue grill. Turn the grill on to medium-high heat (175-190°C/350-375°F) and wait until you see smoke.
Line a roasting rack with foil and place it above the pan of wood chips. Arrange your fish pieces on the rack, leaving space between each piece.
Lower the heat to medium (150°C/300°F) and cover the grill. Let the fish smoke until you no longer see smoke coming from the grill, about 45-60 minutes.
Once the smoking stops, keep the grill covered for 10 more minutes to finish cooking.
Your tinapa should now be golden-brown and firm to touch. Remove from grill and let cool slightly before serving.
For best results, serve hot with garlic fried rice and eggs. Store any leftovers in an airtight container in the refrigerator for up to 5 days.
Note: If your fish tastes too salty, reduce brining time to 4 hours next time. If you want smokier flavor, add more wood chips or extend smoking time.