How to Make It
Heat a large cast-iron skillet over medium-high heat. Coat pan with cooking spray. Add half of okra to pan, cut side down; cook 4 minutes. Turn; cook 1 minute. Remove okra from pan. Repeat procedure with cooking spray and remaining okra. Sprinkle with 1/4 teaspoon salt.
Arrange tomato slices in a single layer on a rimmed baking sheet; sprinkle with 1/4 teaspoon salt. Let stand 5 minutes.
Combine remaining 1/4 teaspoon salt and yolk in a medium bowl, stirring with a whisk until pale yellow and creamy. Stir in vinegar and mustard. Slowly add canola oil in a thin, steady stream, stirring constantly.
Arrange tomatoes on a platter so they overlap slightly; top with any juices that have accumulated on the pan. Top tomatoes with okra and onion; drizzle vinegar mixture evenly over top. Sprinkle with basil, parsley, and black pepper.