Peel the potatoes and zucchini and cut into small cubes. Cut the beef into very small cubes. With beef, you may be able to use very lean minced meat.
Cover the potatoes, zucchini and beef with the water in a saucepan and simmer for about 20 minutes or cook in a pressure cooker for about 8 minutes. Cooking times may vary depending on the size of the pieces. Then puree everything with the cooking water (the amount of water can vary).
You can add the oil before pureeing or, as is currently recommended, stir 1/3 tablespoon per serving into the mash after heating.