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1
Warm the olive oil in a 24–26 cm heavy-based frying pan over medium-low heat. Add the thinly sliced onions and a pinch of salt. Cook, stirring occasionally, for a full 30 minutes until the onions are golden, translucent, and completely sweet — no raw or sharp character should remain, though they must not darken to a deep caramel. Remove from the heat and allow the onions to cool in the pan for several minutes.
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2
Beat the eggs with the grated Parmigiano and the fresh marjoram leaves. Season lightly with salt, bearing in mind that the cheese is already saline. Scatter an additional tablespoon of Parmigiano directly over the cooled onions in the pan, then pour the egg mixture evenly over them.
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3
Return the pan to the lowest possible heat and cover with a lid. Cook for 8–10 minutes until the edges are fully set and only the very centre of the surface retains the faintest tremble when the pan is gently shaken.
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4
Slide the frittata onto a large flat plate. Invert the pan over the plate and, holding both firmly, flip so the frittata returns to the pan with its uncooked side down. Cook uncovered for a further 3–4 minutes over residual low heat until the underside is lightly set and dry to the touch.
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5
Slide the frittata onto a board or serving plate and allow it to rest, uncovered, until it reaches room temperature — at minimum 15 minutes. It will firm sufficiently to slice into clean wedges only once fully cooled; a hot frittata is too tender to hold its shape under the knife.