Preheat oven to 400 F and line 2 baking sheets with parchment paper.
Cut the ends off the zucchini (3) and slice in half lengthwise. Use a spoon to scoop out the seeds and some of the flesh (enough room for your meatballs).
Place zucchinis on one of the prepared baking sheets and brush with olive oil.
Combine Italian sausage (1 pound), egg (1), and grated parmesan (1/2 cup) together in a large bowl using your hands until the ingredients are well incorporated.
Roll the meat into 24 small meatballs, and place them on the second prepared baking sheet.
Place both pans in the preheated oven, and bake for 16-18 minutes, you'll know they're done once the internal temp of your meatballs reaches 165 F, and the zucchinis are fork-tender to your liking.
Meanwhile, heat your spaghetti sauce (1 cup), *I like to do this in a large pan*.
Remove liquid (if any) has pooled in the center of each zucchini boats.
Spoon sauce over each zucchini boat, then 4 meatballs, followed by another layer of sauce. Place 2 cheese slices of provolone halves over the top of each sub. Repeat until you've used all the ingredients.
Bake in oven just until the cheese melts. Enjoy!