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1
Slice or rough-chop all onions. Heat a wide, heavy pot over a moderate flame. Sear the whole piece of beef on all sides until a deep crust forms. The surface should be uniformly mahogany before the beef is removed from the pot.
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2
Return the beef to the pot. Add the onions, carrot, celery, and the small amount of tomato if using. Pour in the white wine. Cover the pot and reduce the heat to the lowest possible setting.
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3
Cook covered for the first two hours. During this period the onions release their liquid and the pot contents will appear watery and unpromising — this is expected. The smell is mild and faintly sweet.
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4
This is where the dish lives or dies. From the third hour onward, as the water evaporates and the onion sugars concentrate, monitor the heat with vigilance: it must remain at the absolute lowest setting. The onions will begin to liquify, then colour, then condense. The target is a deep golden-amber paste — sweet and dense — not brown-black, which signals burning and irrecoverable bitterness. Stir occasionally to ensure no catching at the base. The process continues until hour four to six.
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5
When the onions have completely dissolved into a dense, amber-brown paste that clings to a spoon and the beef is fork-tender, the sauce is ready. Remove the beef whole and set aside for the secondo.
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6
Cook the pasta in well-salted boiling water. Drain and dress generously with the onion sauce, turning until every surface is coated. The sauce should cling and not pool at the base of the bowl.