In a medium pot add water and bring to a boil over high heat. Stir in uncooked rice and 1/8 tsp salt. Place lid on pot and lower heat to a low setting. Cook for 15 minutes (par-cook). Turn off heat.
In a large skillet over medium high heat break up ground beef & cook with diced onion until ground beef is no longer pink and onion has softened down. Drain off excess grease.
Add garlic and cook for 30 seconds. Add Worcestershire sauce, Italian seasoning, 1/2 tsp salt, 1/4 tsp black pepper, 24 oz marinara sauce, 1 cup cooked rice, 1/2 cup Italian blend cheese. Stir well to combine and turn off heat.
Wash & dry bell peppers. Cut tops off of the peppers. Remove stems, seeds, and membranes from inside peppers.
Spray slow cooker with non-stick cooking spray. Pour remaining 8 oz of marinara sauce into bottom of crockpot. Stuff peppers with ground beef & rice mixture and place upright in slow cooker.
Place lid on crockpot & cook on low for 3 1/2 - 4 hours or on high for 2 1/2 - 3 hours. Make sure peppers are fork tender before next step. (They may need extra time to get as soft as you like them.)
Top each pepper with a mound of cheese. Place lid back on slow cooker and cook on high for 5-10 minutes until cheese has melted on top. Sprinkle with chopped parsley before serving.