Cook the bulgar wheat in 600 ml (1 pint) of the stock in a covered saucepan for 10 minutes.
Lay each pepper on a chopping board and make a cut from the base up towards and around the stem, opening out enough to remove the core and seeds but not so much that the pepper splits in two.
Drain off any excess stock from the cooked bulgar, add the sultanas, allspice, feta, a little of the basil and some seasoning. Mix together and spoon into the peppers. Transfer to a roasting tin.
Fry the onion in 1 tablespoon of the oil for 5 minutes until lightly browned. Add the garlic, tomatoes, remaining stock and a little seasoning. Spoon around the peppers, sprinkle with a little more torn basil then leave to cool. Cover and chill until required.
When ready to serve, drizzle the peppers with the remaining oil and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes or until the peppers are softened. Spoon into shallow bowls and garnish with the remaining basil.