This recipe could not be easier. And the best thing is, if you want to sub an ingrendient, it's so easy. No cooked rice on hand? Use some orzo pasta. Don't have mixed beans? Use a can of black beans, or add more vegetables. Add an onion, add shredded cheese... the options are so limitless.
Here is my basic recipe for vegetarian stuffed peppers. I like to use lentils alot. I feel like they have a meaty flavor and I can typically pass them off as a meat option to my family when I mix them in recipes.