Preheat your oven to 350 degrees F.
Prepare your peppers by cutting off the stems and slicing a slit down the middle of each. Inside will be small bunches of seeds. If you like a lot of heat, leave the seeds inside. If you prefer it a little more mild, remove the seeds.
Divide the ground sausage into 6 separate, equal portions and stuff one portion into each pepper. It's okay if thee sausage is exposed and doesn't completely fill the contents of the pepper.
In a baking dish, take 1/4 of the tomato sauce and spread it along the bottom of the baking dish. Add the water and mix it together with a spoon or whisk.
Lay each pepper, sausage-side-up in the baking dish.
Top each pepper with a small amount of the remaining tomato sauce
Cover the baking dish with foil and pop it into the pre-heated oven for roughly 40 minutes.
At 40 minutes, remove the baking dish from the oven and test the peppers firmness. If a fork easily pierces the pepper, they're ready for the next step.
Top each pepper with a little bit of mozzarella cheese.
Turn the oven to broil and put the peppers back in the oven, uncovered.
Broil until the cheese is golden brown and remove from the oven.
Let cool for at least 10 minutes before serving. Enjoy!