Put the lentils in a saucepan and cover with 450 ml water. Add the garlic clove and 1/2 of the onion.
Bring to boil and cover with a lid. Let simmer for 20-25 min until the lentils are cooked but still have a slight bite to them. You do not want to end up with a lentil mush.
Prepare the celery and apple now. Finely chop the other half of the onion too. Once you have chopped the apple, put it in a bowl with water and lemon juice, otherwise the apple will start turning brown.
Drain the lentils and cool down under cold, running water.
Put them in a bowl, add the apples, tomatoes, onion, celery and chopped mint. Drizzle with olive oil and lemon juice to taste. Season with salt and pepper.