Skin, cube (~1-2in) and boil the sweet potato until tender, ~15 mins.
Meanwhile, hollow out the peppers and tomatoes (from the tomatoes, save a bit of the juice/seeds). For the tomatoes you can do this by gently using knife and spoon to loosen the insides, and then spoon out them inside. Save the vegetable tops because you’ll need them later when cooking. Set aside.
In a large skillet and medium-high heat, sautee the onion with a dash of olive oil until translucent. Then add the garlic, thyme, oregano, pepper flake, and a dash of salt and pepper. Add more olive oil if needed. Then add the tomato juices and dill. Saute until the dill has cooked down and no excess liquid remains.
Preheat the oven to 350F/180C.
In a large bowl, combine the cooked potatoes and onion-dill-herb mixture. Use a fork or spoon to loosely combine and mash the potatoes a bit. If using feta, add here but be sure the mixture is cooled to room temperature so the feta doesn't melt into the filling.
Fill the peppers and tomatoes with the filling and place in a large baking pan or glass that has at least a few inches on the sides. Put the tops of the vegetables back on, add ~1.5 inch of water in the pan and cover with foil. This will prevent burning, and will steam the vegetables. Bake for 25 minutes, then uncover, top with a dash of olive oil and bake for another ~8-10 minutes turning on the broiler to low the last ~5 minutes. When the vegetables start to brown on top and are tender, they are ready. Serve topped with toasted nuts, seeds, grains. Yogurt is a nice addition too!