How to Make It
Preheat the oven to 350°. On a rimmed baking sheet, toss the ciabatta cubes with 1/4 cup of the olive oil and season with salt and pepper. Spread the cubes in an even layer and bake for 15 minutes, tossing once, until golden and crisp. Let the croutons cool completely.
In a small bowl, whisk the vinegar with the sparkling water and the remaining 2 tablespoons of olive oil. Season the vinaigrette with salt and pepper.
In a large bowl, toss the romaine with the arugula, radishes and mint. Add the vinaigrette and toss to coat. Fold in the croutons and feta and season with salt and pepper. Serve right away.