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1
Soak the split fava beans in cold water for 2–3 hours. Drain. Do not exceed this window — prolonged soaking causes the beans to lose structure and turn gluey rather than silky.
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2
Place the drained favas in a heavy-based pot with 1.2 l of cold water and the Pecorino rind. Bring slowly to a bare simmer over low heat. No higher — favas scorch easily on the bottom without constant attention.
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3
Cook at the lowest possible heat for 2 full hours, stirring regularly to prevent the base from catching. This is where the dish lives or dies: insufficient time or excessive heat will produce neither the characteristic silky density nor the slightly rough, stone-milled texture that defines marò — the beans must dissolve entirely of their own accord into a dense purée.
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4
Remove and discard the Pecorino rind. Add the wild fennel fronds — or fennel seeds — in the final 10 minutes of cooking only. Their anise notes are volatile; earlier addition drives them off. The soup should carry a faint, clean anise scent.
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5
Remove the pot from the heat entirely. Stir in the grated Pecorino Sardo off the heat — adding it to the soup while still over the flame causes it to become stringy. Taste, then season with salt only now; Pecorino is very salty and the dish can over-salt without notice.
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6
Ladle into bowls. Drizzle each portion with a generous quantity of raw olive oil — the grassy, peppery bitterness of Coratina is the correct contrast to the sweet favas. Serve immediately with carta di musica on the side.