Heat a pressure cooker, add oil and then butter. (I used salted butter.)
Add sliced onion. (Ratio of potato to onion should be 1:3. You can decide how many onions to use depending on their size.)
Sauté for a minute and then add sliced onion. (Garlic shouldn't start to turn brown.)
Sauté for 3-4 minutes till onion turns translucent and then add potato.
Add black pepper powder. (Freshly ground will taste even better.)
Sauté for 3-4 minutes and then add a cup of water.
Cook on high flame until the first whistle. (Shouldn't take more than 3-4 minutes.)
Allow the pressure to settle and then remove the lid.
Blend the mixture ensuring that no lumps remain. (If any plastic part of the blender is going to come in contact with the mixture, allow it to cool and then blend.)
Add half cup water to the mixture and heat it to bring it to a boil. (The soup should be thick, so start with half a cup initially and then add some more if needed.)
Meanwhile, add salt and finely chopped spring onion blades while the soup comes to a boil. (If you've used salted butter, add less salt at this stage. You can taste and add it later if needed.)
Lower the heat and add fresh cream.
Stir in the cream and turn off heat.