Remove fat and tubes from the kidneys and cut into slices. Heat the olive oil moderately in a small pan and briefly fry the kidney slices on both sides. Before turning, add the pressed garlic clove and the rosemary sprig and finish frying the kidneys, this is very quick.
Melt the butter, toss again briefly, remove the kidneys from the pan and season with salt and pepper.
Briefly fry the halved cocktail tomatoes in the remaining roast, then deglaze with the balsamic vinegar. Bring to the boil for a short time, briefly heat the kidney slices again in the sauce and serve immediately with a piece of crispy ciabatta.