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1
Dissolve the yeast in the warm water with a pinch of the sugar. Leave until a foam develops on the surface — the yeast is active.
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2
Combine the flour, remaining sugar, and salt in a large bowl. Add the yeast liquid and olive oil. Work together to a smooth, slightly tacky dough. Knead for eight to ten minutes until the dough is supple and springs back when pressed with a finger.
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3
Place the dough in an oiled bowl, cover, and leave to rise until doubled in volume. The dough is ready when a finger pressed into it leaves an indent that does not spring back.
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4
Heat the oven to 200 °C. Oil a large baking tin generously. Divide the dough into two equal pieces. Press or roll the first piece directly into the tin to form a thin, even layer covering the base.
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5
This is where the dish lives or dies. Press half the grapes firmly into the lower layer of dough — they must be embedded, not merely resting on the surface. Lay the second sheet of dough over the top and press it down to enclose the grapes. Press the remaining grapes firmly into the upper surface with equal insistence, embedding them. Scatter the sugar and rosemary leaves across the top. Every grape must be anchored: grapes that sit loose will not burst into the dough.
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6
Bake at 200 °C until the top is deep golden and the grape juice has caramelised and darkened around each burst grape — the surface should show dark, lacquered rings of concentrated juice. A pale schiacciata is under-baked; the juice must have had time to concentrate and the sweetness to balance the tannin of the skins.
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7
Remove from the oven and allow to cool until warm but handleable. The interior grape pockets will release their juice on cutting — the alternation of bread and hot juice is the characteristic pleasure of the dish. Eat the same day.