Preheat your oven to 350 degrees. Line a rimmed baking sheet with parchment paper.
Scoop out the insides of the zucchini with a spoon or melon baller, leaving about 1/4 inch around the outside of the zucchini. Chop up the zucchini that you pulled out.
Combine half the chopped zucchini (see note), bread crumbs, sausage (raw), cheese, eggs, parsley, 2 minced garlic cloves, salt and pepper in a large bowl.
Fill the zucchini with the mixture, place onto the prepared baking sheet, and bake for 40 minutes.
While the zucchini cooks, make a sauce. Heat oil in a sauce pan. Add 2 minced garlic cloves and sauté for about 30 seconds. Then add in the crushed tomatoes. Bring to a boil, and then reduce to a simmer. Add in the basil, oregano, salt, and pepper. Continue simmering for 15 minutes. At the end of the simmering time, taste the sauce. If it seems a little tart, add a teaspoon of sugar. This will even out the acidity of the canned tomatoes without making the sauce sweet.
After 40 minutes, top the zucchini with the sauce and mozzarella cheese. Put it all back in the oven for 10 to 15 minutes.