Wash the fillet, dab dry and cut into strips. Peel and finely chop the onion. Wash, clean, quarter and seed the tomato. Cut tomato flesh into small cubes. Heat oil in a pan and fry the meat in it until golden brown all around.
Season with salt and pepper. Remove from the pan and set aside. Add to the cooking fat and fry. Add diced tomatoes to the onion, steam briefly and deglaze with vinegar and 3 tablespoons of water.
Add the meat and marinate in it. Season to taste with salt and pepper. Leave to cool a little. Clean and wash the salad and cut into fine strips. Arrange on plates together with the meat and vinaigrette and garnish with parsley if desired and serve.