I used ground turkey instead of beef, and I substituted a blend of pearl couscous, orzo, quinoa and lentils for the rice.
There was a lot of filling left over after I stuffed the peppers, so I decided to turn that into a stuffed pepper soup the next day. I sautéed the tops I cut off the peppers with some onion and mushrooms, added some more seasonings, a can of tomatoes and 2 cups of broth, along with a little cabbage I had in the fridge.