Curry Sauce: In large deep skillet, melt butter over medium heat; cook shallot, stirring, until softened, 2 minutes. Add wine and vinegar; bring to boil over high heat. Boil until syrupy, about 5 minutes. Add stock and bring to boil; boil hard until reduced to 1 cup, about 30 minutes. Add cream; boil until thick enough to coat back of wooden spoon, about 5 minutes. (Make-ahead: Set aside for up to 30 minutes; rewarm.) Whisk in curry paste and tarragon.
Barley Risotto: In saucepan, bring stock and wine to simmer; maintain at simmer. Meanwhile, in separate saucepan, heat half of the oil over medium heat; cook onion and garlic until softened. Stir in barley. Add hot liquid, 1/2 cup at a time, stirring until liquid evaporates and adding more until all liquid is absorbed and barley is tender, about 45 minutes. Add pepper and salt. (Make-ahead: Cover and set aside for up to 20 minutes; rewarm if necessary.) With fork, stir in remaining oil.
Meanwhile, in large ovenproof skillet, heat oil over medium-high heat. Season char fillets with salt and pepper. Place, skin side down, in skillet; cook for 5 minutes or until skin is crispy. Transfer to 450°F (230°C) oven; roast for 5 minutes or until fish flakes easily when tested.