Melt 2 tablespoons butter in saute pan. Add onion and saute for 10 minutes or so over medium heat, stirring often. After onions turn translucent, stir only when bottom begins to brown, scraping up browned bits, and allow to brown again. (You may have to cover the pan for a bit if the onions start to dry out) Repeat until onions take on a deep golden color and are quite soft. They will have reduced considerably.
Meanwhile, add to a medium bowl the beef, rice, slightly beaten egg, seasoned salt and ketchup.
When onions have cooked and cooled slightly, add them to the beef mixture. Combine well with wooden spoon or by hand.
Wash green pepper, dry, slice the very top off and clean insides of seeds and membrane.
In medium stock pot sized to hold the peppers you've filled, add can of diced tomatoes and can of tomato soup (no water yet). Stir to combine. Bring to a slow boil over medium heat.
Fill peppers with beef mixture to within a 1/4" of top. Add to pot all standing upright. Add enough water to cover the peppers at least by 1/3, but no more than a soup can full. Cook covered on high simmer for about 30 minutes, then move the peppers around to balance the cooking speed, or onto their sides if there's room. Cook for at least one hour total and up to an hour and a half.
Serve on a plate, split in half covered with diced tomato and some sauce. Crusty french loaf or rye bread is a great side as well as mashed potatoes. You can also spread this meal farther by adding kielbasa chunks to the cooking pot. Another favorite addition is a can of sweet kraut along with the tomato and soup.