For the balsamic dressing: In a small bowl, combine the balsamic vinegar, mustard, zest, garlic and lemon juice. Whisk in the olive oil until emulsified. Season with salt and pepper.
For the fried okra: Add okra pieces to a medium bowl. Drizzle just enough buttermilk to coat the okra and toss together. Season with salt and pepper. To a separate bowl, add flour. Drain the okra and add to the flour bowl, toss the okra to coat each piece with flour. Shake off any excess flour. Fry at 350 degrees F until golden brown, for 2 to 3 minutes. Remove to paper-towel lined plate to drain.
For the salad: In a large bowl, combine the baby spinach, iceberg lettuce, tomatoes, bell peppers and onions. Toss with the balsamic dressing and top with the fried okra croutons.