Steam the Brussels sprout pieces in a double boiler till quite soft, but still bright green. Place on a clean food tray, to spread out and stop the steaming. Set aside.
Take a heavy saute pan, heat some oil and gently sweat the garlic and onion until soft. Add some salt and a little water to allow the garlic to steam slowly. When the vegetables are soft, add the chopped hazelnuts, the potato pieces and the ham. Mix well.
Add the soft Brussels sprouts. Taste for seasoning, adding more sea salt and pepper if you want to. Then place a lid on the pan, and set aside somewhere warm.
Take the 4 slices of bread, drizzle with a little olive oil on both sides, and toast under the grill. Alternatively, you can just toast the bread without olive oil in a normal toaster, and then drizzle a little olive oil on top.
Place a toasted bread slice on each of four serving plates. Spoon the Brussels sprout mixture on top, generously. Sprinkle on the grated Parmiggiano Reggiano and oregano on top and serve.