I used to make stuffed peppers all the time when I was vegan. I haven't had them in over two years now mostly because I cut out grains and didn't know what to stuff them with.
I came up with a good solution. Use zucchini instead!
bell peppers, washed and seeded
1 medium onion, diced
Cut and see the peppers and set aside.
Cook the ground beef and drain off the extra fat. Add the onion and saute for a minute then add the spaghetti sauce. You want this to be thick so go easy on the sauce. Optionally, add some cheese to the mix and maybe some chilli powder. After it is cooked down a bit add the shredded zucchini and cook for a minute or two.
Stuff the peppers with your meat mixture and place in an oven pan. I like how it looks to put the top back on. Bake at 350 till the pepper is as done as you like it. Mine took 45 minutes. Take out of the oven and sprinkle with cheese when there is about 5 minutes lift. cook the remaining time.