Cook rice and let cool enough to touch. (can cook rice the day before and use straight from the fridge)
Preheat oven to 350 F and position rack in the center.
Half the bell peppers lengthwise and clean out the cores and seeds. Set aside.
In a large bowl, combine rice, herbs, spices, and 1/2 cup of spaghetti sauce.
Add ground beef and egg. Mix thoroughly (hands work best, or use the paddle attachment in a stand mixer).
Spread the rest of the sauce (about 2 1/4 cups) into a 9x13" baking dish and arrange the pepper halves on top.
Spoon the filling evenly into the peppers, pressing down as needed. All of the filling should be used.
Top each pepper with 1 Tbsp of feta cheese.
Cover the dish with foil and bake for 50 minutes.
Uncover and bake another 10-15 minutes, until the filling reaches 165 degrees F.