Sauté on low for 7-10 minutes, preferably with a lid that fits loosely over the cooker (it can be the lid of one of your hob pans).
Deglaze well, this means scraping the bottom of the pot with a bit of liquid (it can be some of the stock) and a wooden spoon; to remove any caramelised burnt-on bits (which will add flavour to the soup). Deglazing helps your pressure cooker reach pressure.
Stir in the cold stock and the lentils.
1.2 litres stock, 300 g dried red split lentils
Make sure the sealing ring is properly in place. Steam release set to Sealing.
Pressure cook, high pressure, for 2 minutes.
At the end of pressure cooking, leave it to do a full natural pressure release, ie let the float valve pop back down of its own accord if you want to make your life easier. It will take 20-25 minutes.
You can also vent the rest of the steam trapped inside after about 15 minutes.
Or leave it on warm as long as you need to.
When you open the lid, give it a good stir and see how it turns into a lovely blended soup with no effort