Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Measure thickness of fish fillets; set aside. Preheat the oven to 425 degrees F. Coat a 3-quart rectangular or oval baking dish and a large baking sheet with nonstick cooking spray; set aside.
In a large bowl, combine water, 2 tablespoons brown sugar, and 1/4 teaspoon salt; add sweet potatoes, tossing to coat evenly. Layer sweet potato slices in prepared baking dish. Bake, uncovered, about 30 minutes or until tender. Remove from oven. Cover to keep warm. Increase oven temperature to 450 degrees F.
Meanwhile, place flour in a shallow bowl. In another shallow bowl, combine egg and the water; beat with a fork. In a third shallow bowl, combine pecans, flaxseeds, 2 tablespoons brown sugar, and 1/4 teaspoon salt.
Dip each fish fillet in flour, turning to coat both sides and shaking off excess. Dip in egg mixture, then coat generously with pecan mixture. Place fish fillets on the prepared baking sheet. Bake in the 450 degrees F oven until fish flakes easily when tested with a fork. Allow 4 to 6 minutes per 1/2 inch thickness.