Add 2 Tbsp olive oil to the inner pot and turn the Ninja Foodi on high sear/sauté. Place your sausages in the inner pot and sauté until brown, flipping them as needed. Allow them to cook in the inner pot while you cut your onions.
Slice the onions in ¼ - ½" slices and place in the inner pot. Sauté for about 5-10 minutes or until they start to brown. Put the pressure lid on and turn the seal to VENT. Decrease the heat to medium and allow to cook for 7-10 minutes. This will ensure that the sausages are cooked at least 75 - 80% of the way done. This is important because if the sausages are not cooked enough before the PC time, they will not be fully cooked.
Remove the pressure lid. Mix about 1-2 Tbsp of beef stock with the 2 Tbsp of cornstarch and stir to make a slurry. Deglaze the pot with the remaining beef stock and scrape the bottom to remove any brown bits. Add the salt, pepper, and thyme. Stir to combine. Add in the cornstarch slurry. Stir. Cook on medium heat until the sauce thickens slightly. Turn the Ninja Foodi off.
Remove the Sausages and place them in a Fat Daddio 8"x2" pan. Pour the onion gravy over top. Wash the pot and return it to the Ninja Foodi base.
Place the basket diffuser legs in the bottom of the pot with the pointed side of the legs pointing up. Add 2½ cups of water. Peel and dice your potatoes to about 1½ - 2" dice and place in the pot. Try to get them in the open areas between the legs so your 8" pan can fit in the center.
Place the pan with the sausages and onion gravy into the pot, resting on the diffuser legs. Cover with foil or a silicone cover. Place the frozen peas into a 6"x2" Fat Daddio Pan and cover with either foil or a silicone cover. Place the peas on top of the 8" covered pan.
Put the pressure lid on and turn the valve to seal. Pressure cook on high for 10 minutes. When the time is up Immediately release the pressure.
Carefully remove the peas. Add 1 tbsp of butter and a bit of salt, cover until ready to serve. There will be liquid on top of the 8" pan, blot it off or carefully tip the cover so the liquid goes down into the inner pot and not into the gravy. Remove the pan. I found the best way to do this is by gripping the side with rubber tipped tongs and pulling it straight up. Keep covered.
Remove the diffuser legs and scoop out the potatoes with a slotted spoon and put into a mixing bowl. Using a hand mixer (turned off), slightly mash the potatoes with the beaters to allow the steam to escape. Add 6 Tbsp of butter, 1 tsp of salt, and ¼ cup of heavy cream or half and half. Turn the hand mixer on and mash on low until creamy. Add more cream or half and half if needed. Salt and pepper to taste.