Heat the oil in a pressure cooker. Add onions and garlic and cook till the onions turn translucent.
Next add the tomatoes, chili powder, ground cumin, ground coriander, salt and pepper. Cook till the tomatoes are mushy.
Add lentils and 21/2~3cups of water. Close the lid and cook for 2~3 whistles. Let the pressure come down naturally.
Open the lid and whisk the lentils until smooth. If the soup looks too thick, add some water and simmer until heated through.
Taste and adjust the seasoning. Stir in the lemon juice.