Finely slice the onions and gently cook with a glug of oil and a pinch of salt in a frying pan for 20 mins.
Heat oven to 200C/180C fan/ gas 6. Put 2 of the wraps on a baking tray and divide the Gruyère between them. Top each with another tortilla, then the onions. Halve the olives and scatter on top with the anchovies. Sprinkle over the remaining Gruyère and bake for 5-10 mins until crisp.