Prepare your pot and your stacker pans first. Make sure your metal inner pot is in the base of your pressure cooker. Pour in 1 cup of water in your 6 Qt pot. (if you have an 8 Qt add 1½ cups of water)
To each stacker pan add ½ can of diced tomatoes and try to make sure the juice is evenly dispersed between the pans. Set these pans aside for later.
In a 1 gallon zip lock bag add: cottage cheese, parmesan, eggs, salt, pepper, oregano. Now, remove air from bag and close it shut. Squish that up to mix it up well. Squeeze the filling into one corner of the bag and get ready to use the bag for piping into the shells.
Cut the corner off your bag and pipe the filling into each shell until each is full. Place each shell into the stacker pan right on top of the diced tomatoes.
You can set up to 6 shells into each stacker pan.
Top both pans of shells with spaghetti sauce. Just pour half of the jar onto each stacker pan. Then add ½ mozzarella cheese to each pan.
Place stacker pans in holder and cover loosely with foil and lower in pot. DO NOT use the lid that comes with your stacker pans. That keeps out too much heat and steam.
Set pot on Pressure Cook Manual for 30 minutes and QR. Then you need to remove the stacker pans and switch them around (top pan now goes on the bottom). Place foil on top and set your Pressure Cook time to 20 and this time you will let your pot NPR. This will ensure that your shells are cooked through. I have tried this recipe several times and if you do two pans, this is the only way to get them to cook evenly. On the flip side, you don't have to precook your pasta.
Be sure to test a shells to make sure they are done. You can prick them with a for and feel the resistance. If NOT, add an additional 5-10 minutes.