Wash the potatoes and cook covered for about 20 minutes. Quench, peel and let cool down a little. Cut potatoes into thick slices.
Wash the spinach and drain well. Wash and quarter the tomatoes. Peel garlic and cut into fine cubes.
Heat 1-2 tablespoons of oil in a large coated frying pan. Fry 1/3 each of the potatoes, garlic, tomatoes and spinach at high heat. Season with salt and pepper and place in a rectangular, greased casserole dish (approx. 20 x 25 cm).
Fry the other vegetable ingredients in the same way and put them into the dish.
Whisk eggs and milk. Season with salt and pepper and pour into the oven dish. Let it stand in the preheated oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3) for 35-40 minutes.
Let the tortilla cool down a little and cut into small pieces.