Heat oil in a medium nonstick skillet over medium-high heat. Add shallots; sauté 3 minutes. Remove from heat.
Place shallot mixture, ginger, and next 5 ingredients (ginger through garlic) in a food processor; pulse until coarsely blended. Set aside.
Arrange watercress on a large sheet of foil. Arrange fillets on top of watercress; spread shallot mixture over fillets. Fold foil over fish to form a packet; loosely seal. Place packet in the bottom of a bamboo steamer. Cover with steamer lid. Add water to a large skillet to a depth of 1 inch; bring to a boil. Place steamer in pan; steam fillets 15 minutes or until fish flakes easily when flaked with a fork.