Preheat oven (electric cooker: 220 °C/circulating air: 200 °C/gas: see manufacturer). Line baking tray with baking paper. Wash parsley and chop finely. Wash and dry the lime, grate the peel. Squeeze lime. Cut the chilli lengthwise, remove seeds, wash and chop. Mix everything with 2 tablespoons mayonnaise. Season with salt. Cut roast beef into strips and mix with it.
Peel onion and cut into rings. Spread the slices of bread with 2 tbsp. mayonnaise. Add the roast beef mix and cover with onion rings.
Spread the sandwiches on the tray and roast in a hot oven for 8-10 minutes.