This recipe moves very fast, so I strongly recommend having the peppers and other ingredients prepped and measured out before you start sautéing ground beef. Dice the onion into about ¼" dice. Mince 2-4 garlic cloves until you have about 2 teaspoons. Make sure your peppers will fit on the rack in the low position with enough clearance for the pressure lid to go on. Cut the tops off of the peppers and remove the stems from the middle of the tops. See post for tips if needed. Place the peppers upside down on the rack with the tops resting on the bottom of the pepper to avoid liquid from collecting in the pepper during the steam time.
Turn the Ninja Foodi on and select the sear/sauté function on high. Add the ground beef and break it up into small pieces. Once the ground beef starts to brown, add the diced onion, minced garlic, and the seasoning blend. Stir and continue to sauté until the ground beef is 50-75% cooked and broken up into small pieces for the filling. This takes about 5 minutes.
Add the uncooked rice, Worcestershire sauce, balsamic vinegar and stir to combine. Deglaze the pot with 1¼ cups of beef stock and make sure to scrape the bottom of the pot to remove anything that may be stuck on the bottom.
Place the rack in the low position with the peppers into the pot over the meat/rice mixture and put on the pressure lid. Turn the pressure valve to VENT and select the steam function for 5 minutes.
When the time is up and the pin has dropped, immediately open the lid and remove the rack with the peppers. Set the peppers aside to cool so you can handle them.
Make sure the rice is cooked the way you like it and that there isn't too much liquid left. If you need to cook the rice longer, use the sear/sauté function on low or put the pressure lid back on and let it sit for 5-10 minutes. Add 1 cup of marinara or spaghetti sauce and stir. Taste and adjust seasonings as needed.
Fill the peppers with the meat/rice mixture and top with shredded cheese if using. Place the tops back on the peppers and lower the rack back into the Ninja Foodi inner pot. No need to clean the pot before this step.
Pour additional marinara over the peppers if desired and lower the crisping lid. Select broil (grill in the UK) for 10 minutes. Check the peppers after about 4 minutes to see how the cheese is doing. Continue to broil until the cheese is melted and beginning to brown. This usually takes about 7-8 minutes, but keep an eye on it.
Remove the rack with the peppers from the Foodi and Serve. Enjoy!