Sauté the onion in half of the oil until tender. Add chopped tomatoes, garlic, tomato juice, salt and pepper. Cook over low heat for 15 minutes. Skin and slice the pompano into scallopines. Flatten with a mallet. Season with salt, pepper, and lemon juice. Dredge pompano in flour and sauté briskly in remaining oil. Cook noodles in salted boiling water. Drain, and combine with velouté sauce. Arrange on preheated serving platter. Top with pompano slices. Spoon hot tomato and onion sauce over fish. Sprinkle with parsley. Serve hot.