Preheat oven to 375 degrees. In a large non-stick skillet, Lightly brown beef and onion. Turn heat to low and add seasonings and cook until beef is completely browned. Remove from heat and stir in spinach.
Wash bell peppers and remove tops, cores and seeds. Stuff each pepper with 1/2 to 2/3 cup of beef mixture. Put peppers in glass baking dish and pour 2 tbsp water into the bottom. Cover with foil and bake for 50 minutes.
Remove stuffed peppers from oven and take off foil. Turn up oven to 400 degrees. Cook peppers for additional 10 minutes. Allow to cool somewhat before serving.