Very quick lentil soup

8 ounces split red lentils
2 large onions, peeled and roughly sliced
2 garlic cloves, peeled and chopped
juice of 1/2 lemon
salt and freshly ground black pepper
olive oil
1 teaspoon ground cumin (optional)

Put the lentils, one of the onions, and the garlic into a large saucepan with 4 cups of water. Bring to the boil, then let simmer for about 30 minutes, until the lentils are very tender and pale. Stir well to get a smooth texture. Then add some lemon juice (start with 1 tablespoon) and some salt and pepper until it tastes just right, and serve. If you want to make it spicier, fry the other onion in a little oil for about 10 minutes, until it`s crisp and lightly browned; add the ground cumin and stir for a few seconds longer. Pour this mixture over the top of the soup just before you serve it.