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1
Soak bread in a bowl of water 30 seconds, then squeeze dry and set aside.
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2
Drop garlic into a food processor with motor running, then drop in nuts and process until finely ground. Add bread and 3 tablespoons lemon juice and process until smooth. With motor still running, add oil in a slow, steady stream until incorporated. Transfer to a bowl, then stir in salt and pepper and lemon juice to taste.
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3
Remove any leaves from cauliflower, leaving core intact. Cut cauliflower into 1/2-inch-thick slices as if it were a loaf of bread, then cut slices into 2-inch pieces (some florets will separate).
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4
Bring cauliflower to a boil with enough water to cover it in a 4- to 5-quart pot, then reduce heat and simmer until just tender, 4 to 6 minutes. Drain well in a colander and pat dry.
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5
Heat 2 tablespoons oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then saute half of cauliflower, turning over occasionally, until very well browned on both sides, about 8 minutes total.
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6
Transfer to a platter with a slotted spatula and season with salt and pepper.
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7
Wipe out skillet, then saute remaining cauliflower in remaining 2 tablespoons oil in same manner, transferring to platter and seasoning.
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8
Serve cauliflower, warm or at room temperature, with sauce and celery leaves (if using).
