Preheat the oven to 425 degrees . Cut the crust from the bread. Cut the loaf lengthwise in half. Cut each half lengthwise in half, creating 4 planks.
Arrange the bread in a single layer on a rimmed baking sheet. Bake until toasted, about 5 minutes. Let cool. Turn on the broiler.
Place the cabbage, bay leaf and 1 cup water in a large pot with a lid. Season with salt and bring to a boil. Cover and cook until the cabbage is crisp-tender, 7 to 8 minutes.
Heat a grill pan over medium-high.
Drain the cabbage; transfer to a paper towel-lined baking sheet. Let drain a minute or two, then discard the paper towels. Spray the cabbage with cooking spray; season with salt and pepper and the caraway, coriander and cumin. Transfer to the grill pan; cook the cabbage until grill marks form, 2 to 5 minutes per side.
In a large skillet, melt the butter over medium-high. Add the apples, celery and onion; season. Cook, tossing occasionally, until crisp-tender, 7 to 8 minutes.
In a small bowl, whisk the mustard and syrup. Spread on the bread and top with the apple mixture. Arrange 2 wedges of cabbage on each toast and top with the cheeses. Broil until the cheeses brown and bubble, 3 to 5 minutes.
Top with the herbs and celery leaves, cut each bread slice into thirds and serve.