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1
Melt the butter in a heavy pot over a low flame. Add the leek, celeriac and potato from cold, so the fat warms around them gradually. Cook, stirring occasionally, until the vegetables are fully translucent and yielding but have taken no colour — the slow fat-sweat draws out their sweetness.
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2
Pour in the bone broth. Add the thyme sprigs. Bring to a gentle simmer and cook until both vegetables are completely tender and the broth has absorbed the earthy depth of the celeriac — taste the broth; it should be round, gelatinous and carry the backbone of the bone stock throughout.
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3
Heat the grill to its highest setting. Rub each individual ovenproof bowl with the cut face of the garlic clove. Lay one slice of stale rye bread flat in the base of each bowl. Ladle a small amount of hot broth directly over the bread first, so it absorbs moisture without disintegrating, then ladle the vegetables and remaining broth over the top. The bread should swell and hold its structure — if it collapses to paste the bread was not stale enough.
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4
This is where the dish lives or dies: cover the entire surface of each bowl with a thick, unbroken layer of grated Fontina DOP — it must constitute a full blanket, visually dominant, with no soup visible beneath. Place the bowls under the grill until the Fontina is bubbling across its full surface and carries patches of light brown — any thinner and the cheese merely melts; the dish requires a gratin.