Heat the grill to medium-high.
Whisk all the seasonings together in a small bowl.
Brush onion slices on both sides with oil. Rub one side of each slice with seasoning mixture and place rub side down on grill. (You’ll have some spice rub leftover.)
Grill until golden brown and a crust has formed, 2 to 3 minutes. Flip and continue grilling until just thoroughly cooked, 2 to 3 minutes longer. Serve hot or at room temperature.