Soak the red onions in ice water for 10 minutes and then drain.
Meanwhile, toss the bread cubes with 1 tablespoon of the oil on a rimmed baking sheet and season with salt and pepper. Toast in the oven until the bread cubes are crisp and golden brown, 8 to 10 minutes.
Whisk together the mayonnaise, vinegar, honey, anchovies if using, garlic and lemon juice in a large bowl. Slowly whisk in the remaining 1/2 cup oil. Add the kale, romaine, Parmesan and drained onion to the dressing and toss to combine. Divide the salad among 4 plates and top with the croutons.
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