Combine all ingredients in small saucepan over medium-high heat. Stir until butter and sugar are melted and combined with the other ingredients. Simmer the mixture until it is thick enough to coat a spoon. Remove from heat and let cool to room temperature. Reserve half of the sauce and serve warm with salmon.

Brush the salmon, eggplant, and onion with the sauce on both sides and let them stand loosely covered for 30 minutes.

To prepare the plank.

Soak cedar plank in cold water for at least 1 hour or up to 12 hours. Prepare a 2-zone fire on a charcoal grill. Push all lit coals to one side of the grill. Set the cedar plank on the grill over the hot zone, coverthe grill, and wait until the plank starts to smoke - 2 to 3 minutes. Slide the plank to the cooler zone and arrange salmon on top - thicker ends closest to the hot zone. Place eggplant and onions over hot coals.

Cover the grill (vents open on a charcoal grill) and cook until the fish is almost cooked thru with just a touch of pink in the center (instant read thermometer inserted in thick part to register 135 degrees F, 10 to 14 minutes).

Transfer the fish, still on the plank, to a serving platter and serve with reserved sauce, eggplant and onions.