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RINSE the half leek and chop into thin rings. PEEL the Jerusalem artichokes and keep them in water with a bit of lemon juice to prevent them from discolouring.
PEEL the garlic cloves and finely chop them. FILL a medium-sized pan with water and salt and bring to a boil. CHOP the Jerusalem artichokes into pieces and boil them for a few minutes.
HEAT a large skillet on medium-high heat and fry the bacon lardons for a few minutes. ADD the garlic, leek, thyme and bay leaves and sauté them a few minutes more. POUR in the cream and let it simmer for about 5 minutes more.
Then turn off the heat and let it cool off slightly. PRE-HEAT the oven to 190 degrees celsius or 375 degrees Fahrenheit. DIVIDE the al dente pieces of Jerusalem artichokes over the oven dish.
REMOVE the thyme sprigs and bay leaves. BEAT the eggs with a whisk and ADD the cream mixture as soon as it's cooled off slightly
DIVIDE this mixture over the oven dish and finish it off with grated Parmesan cheese, a handful of panko (Japanese breadcrumbs) and small cubes of butter. BAKE the gratin in the oven for about 30 minutes, or until the cheese is beautifully browned. Serve with a salad, enjoy!