Bring the stock to the boil in a large saucepan. Peel and halve 1 of the garlic cloves, then add to the pan with the split peas and cook for 40 minutes until the split peas are tender. Drain, if necessary, then season with salt and pepper and leave to cool slightly.
Meanwhile, lightly spray a roasting tin with oil. Put the remaining garlic cloves in the tin with the peppers and onion and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes. When cool enough to handle, squeeze the roasted garlic cloves from their skins and chop with the roasted vegetables.
Mix together the split peas, vegetables, mint and capers in a large bowl. Flour your hands and shape the mixture into 12 patties. Cover and chill until ready to cook.
Make the tzatziki by mixing the ingredients together in a bowl, cover and chill for 30 minutes before serving.
Heat a frying pan and spray with oil. Cook the patties, in batches if necessary, for 2 minutes on each side. Serve 3 patties per person, hot or cold, garnished with mint leaves, along with the tzatziki.