1. Preheat the oven to 325 F (170 C, Gas 3).
2. Melt the butter in a saucepan over medium heat, then fry the onion until cooked. Set aside.
3. Put the flour, baking powder, cayenne and cheese in a large bowl. In a separate bowl, mix the milk and egg together, then slowly pour into the flour mixture and beat with a handheld electric whisk until all the ingredients are well mixed.
4. Stir in the onion and spinach with a wooden spoon until evenly dispersed.
5. Spoon the mixture into the paper cases until two-thirds full and bake in the preheated oven for 30-35 minutes, or until deep golden and the sponge bounces back when touched. A skewer inserted in the center should come out clean. Leave the muffins to cool slightly in the tray before turning out onto a wire cooling rack to cool completely.
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