Peel and slice the onions. Remove the stalks from the peppers, halve and discard the seeds. Shred the peppers fairly thinly.
Heat the butter in a frying pan and add the onions and peppers. Cover with a lid and fry gently, stirring occasionally, for about 10 minutes until the onions and peppers have softened.
Serve at once. They are particularly nice with fried or grilled steaks