First, prepare the sauce. Heat a deep pan or dutch oven over medium heat. Add oil. When oil is hot, add half of chopped onion. When onion softens (about 4 minutes), add garlic and saute for another 2—3 minutes, until garlic is slightly golden.
Add tomato sauce to pot, plus ½ cup of flitered water. Season with 1 tbsp paprika, cayenne, plus salt and pepper to taste. Lower heat to smallest flame and cover while you make the zucchini boats.
Cut zucchini in half lengthwise. Scoop out flesh to create "boats." Place flesh on cutting board. In food processor or by hand, finely chop zucchini flesh. Transfer to bowl and add ground beef, mashed potato, and half of chopped onion. Season with 1 tbsp paprika, and salt and pepper to taste. Mix well.
Generously fill zucchini boats with ground beef mixture, packing firmly. When all zucchini boats have been filled, gently place them, one by one, in sauce. Spoon a little bit of the tomato sauce over the zucchini boats. Simmer for 30-40 minutes, or until beef is cooked through.
Serve with grated cheese, if desired.