Brush the wonton wrappers with the vegetable oil and sprinkle with a pinch each of salt and pepper. Cut the wanton wrappers into triangles and bake on a baking sheet until golden and crisp, about 6 minutes.
Heat the butter in a skillet over medium heat, add the onions and cook until softened, about 6 minutes. Stir in the flour, 3/4 teaspoons salt and a couple turns of black pepper until well incorporated. Whisk the milk into the onions and bring to a boil. Stir in the creamed corn and bring to a boil, then remove from the heat. Stir in the char and watercress and let sit until the fish is just cooked through, about 8 minutes. Serve with the wonton crackers.
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