Preheat oven to 400˚F. Line a baking sheet with foil. Cut off the tops of the zucchini, then cut in half lengthwise. Scoop out the insides with a spoon and pat dry with paper towel. If the zucchini does not lie flat, then cut a portion from the bottom so they lie flat.
Place hollowed zucchinis on baking sheet and spray insides with avocado oil spray. Sprinkle with salt and pepper. Add 1 Tbsp. of marinara sauce to each zucchini. Leave a small rim near the edges without sauce.
Top with sliced Fresh Mozzarella, onion and tomatoes.
Bake for about 12-15 minutes, or until zucchini is tender. Remove from oven, garnish with red pepper flakes and basil and serve.