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Heat the oil in a large, deep pan. Add the onions, cover, then gently fry for 15 mins until soft and almost transparent. Add the butter and pine nuts, turn up the heat, then fry for 5 mins more or until the onions are dark golden. Stir in the garlic, grate in a little nutmeg, then fry for 1 min. Splash in vinegar.
Meanwhile, heat the largest pan you have until very hot. Throw in half the spinach, cover for 20 secs, then stir until just wilted. Tip into a colander set over a bowl, then repeat with the second bag of spinach.
To serve, toss the cooked spinach into onion mix and season generously.