Preheat the oven to 200C/400F/Gas 6.
Place one tortilla onto a baking sheet and top with half of the Stilton and half of the onion and season with freshly ground black pepper.
Place another tortilla over the topping and cover with the rest of the Stilton and onion.
Cover with the final tortilla and sprinkle with grated cheddar. Transfer to the oven to bake for 5-6 minutes, or until the cheese has melted.
To serve, cut the quesadilla into four and place onto a plate.
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