Place the halved and cored bell peppers in a 7x11-inch baking dish and bake for 15 minutes, or until softened. Remove from the oven and pat the inside dry.
Make the filling: place the tuna, celery, green onions, peas, yogurt, lemon juice, mustard, hot sauce, salt, and pepper in a medium-sized bowl. Carefully mix until well combined.
Stuff each bell pepper half with stuffing and sprinkle the cheese on top. Bake for 5-6 minutes or until the cheese has melted. Serve immediately.