With a sharp knife, split the tough muscle surrounding each testicle and remove the inner meat.
Discard the muscle tissue and transfer them to a large saucepan.
Cover with water and vinegar. Bring to the boil and simmer for 15 minutes, until tender.
Drain and rinse under cold water until cool.
Slice each testicle into 1cm thick ovals and set aside.
Put the onion, garlic and peppers into a large pan.
Add the olive oil and water.
Cover and cook on a high heat for about 10 minutes, until soft.
Season well and set aside.
Toss the sliced testicles in the flour, followed by the beaten egg and finally the breadcrumbs.
Heat a large pan with the vegetable oil and fry each piece of testicle in a single layer, for 3-4 minutes until golden.
Add the butter, let it sizzle for a minute and stir in the parsley.
Season the testicles and serve with the stewed peppers.