Cook sweet potatoes in the microwave for 5-8 minutes until they are fork tender (flip them half way). Once cooked, scoop and flesh and spread out on a plate or a larger surface are so that it cools faster. I put it in the fridge or freezer for a few minutes.
Bring a large pot of salted water to boil. Add pasta shells and boil for 4-5 minutes (par-boil), drain.
In a large bowl, mix together the cooled sweet potato flesh, ricotta, garlic, parmesan cheese and defrosted/cooked spinach is using.
In a separate bowl, mix the tomato sauce with the water.
In an 8-by-8” pyrex or baking pan, spread a little sauce at the bottom. Stuff each shell with some sweet potato mixture and place on top of the sauce in one layer. Distribute the rest of the sauce over the shells evenly. Sprinkle a little extra parmesan cheese on top.
Bake 350˚F COVERED with foil for 45 minutes, serve hot!