In a large ovenproof skillet over medium-high heat, brown the onions and garlic in the oil. Add the spelt and cook for 1 minute, stirring often. Season with salt and pepper. Deglaze with the wine and let reduce until almost dry. Add the broth and bay leaf and bring to a boil. Cover and cook for 15 minutes or until three-quarters of the liquid has been absorbed and the spelt is cooked but still firm to the bite. Adjust the seasoning. Remove from the heat and set aside.
With the rack in the middle position, preheat the oven to 400°F (200°C).
Meanwhile, in a pot of salted boiling water, cook the cauliflower for 5 minutes or until al dente. Drain and set aside.
In the same pot over medium heat, melt the butter. Add the flour and cook for 1 minute, stirring with a whisk. Add the milk and bring to a boil while stirring. Simmer gently for 5 minutes. Remove from the heat and whisk in ½ cup (35 g) of the Parmesan, the nutmeg and cayenne. Season with salt. Set aside.
In a small bowl, combine the breadcrumbs and remaining Parmesan.
Arrange the cauliflower over the spelt. Pour the béchamel sauce over the cauliflower and sprinkle with the Gruyère and the breadcrumb mixture. Cover and refrigerate at this point, if desired.
Transfer to the oven and bake for 15 minutes or until the cheese is golden (bake for 25 minutes if the gratin was refrigerated). Let cool for 15 minutes before serving with the Kale and Celery Waldorf Salad (see recipe).
Spelt is available in many groceries stores, specialty shops and health food stores. It can be replaced with pearl barley in this recipe.