Prepare vegetables. Put 1 tbsp or so of olive oil into your pressure cooker with sausage dials and saute for about 5 minutes, stirring so they can brown just a bit.
Turn pot off and layer in sliced onions on top, then your cubed potatoes (the smaller they are the softer they'll get, don't make too small or they'll just fall apart), then cubed cabbage squares broken apart. Make sure to not pass the overfill line inside your pot.
Now pour your liter of soda on top of everything, do not stir together.
Close lid and steam valve and set to high for 6 minutes followed by a quick release. (note: it will take a bit for it to reach pressure since there is a lot inside, that is okay)
Do a quick release when done. Lift lid and set to saute again. Scoop 1/2 cup of the hot liquid from the pot into a measuring cup, then add your 1/2 cup of tomato paste, whisk together until smooth. Add this into your pot, stir.
To thicken add cornstarch and a bit of cold water into a dish, just enough to whisk together so it is smooth and not lumpy. Add this and stir continuously until it thickens. Turn pot off.
It will continue to thicken a bit as it cools and sits, serve with rolls with some parmesan cheese on top if desired.