Preheat the oven to 400 degrees F. Make the croutons: Toss the bread cubes with the olive oil and season with salt and pepper. Spread on a baking sheet and toast in the oven until slightly golden, 8 to 10 minutes.
Meanwhile, whisk the ketchup, mayonnaise and chopped gherkins and juice in a medium bowl. Season with salt and pepper and whisk in 1 to 2 tablespoons water to thin slightly, if desired.
Divide the Boston and iceberg lettuce among bowls and top with the scallions, cucumber, roast beef, cheese, cabbage, croutons and whole gherkins. Drizzle each salad with 1/4 cup dressing. Serve with more dressing on the side.