Cook the pearl barley in 300 ml water for about 30 minutes, until tender.
Meanwhile peel the onions and thinly slice them.
When the water is completely absorbed, pour in the vegetable stock, add the onion, lentil and spices.
Bring to the boil and simmer for 90 minutes under cover, stirring occasionally. If necessary, add more vegetable stock.
In the meantime thinly slice the two onions for serving and wrap them in a paper towel to dry out. Leave them for half an hour.
In a frying pan heat some oil and at low temperature gently fry the onion slices until golden. This will take about 15 minutes. Add the sliced garlic and cook for another 5 minutes. Drain them on paper towels and set aside.
When the soup is cooked (i.e. the pearl barley and lentils are soft), season to taste with salt & pepper, then stir in the coriander and lemon juice.
Ladle the soup into bowls and garnish with the fried onion rings. Serve warm.