
Add the tomato purée and bring to a gentle boil. Reduce the heat to a low simmer and cook for 15 minutes. This can be made in advance and stored in a sealed container in the fridge till needed.
Trim the each end of each zucchini. Using a mandolin set at the 1/16 inch setting, cut the zucchini into as many slices as is reasonable. Repeat with all the zucchini.
Alternatively, stand each vertically, and make ideally 1/6-inch thick cuts down to create long slices—these don’t have to be perfect.
In the end you want enough slices to make 12-16 rollups, whether from individual large slices, or doubled up medium slices. Arrange the slices on the prepared baking sheets.
Drizzle the zucchini lightly with olive oil. Season with salt and pepper. Transfer pan to the oven and roast for 6-8 minutes, (double the time if using hand cut zucchini) see Notes. Remove pans from the oven and use a fork or tongs to flip each zucchini slice over, return pans to the oven, and roast for 5 minutes more, or longer if needed. See Notes.
Spoon 1 1/12 cups or so of the tomato sauce into a 9×13-inch or comparable baking dish—the bottom of the dish should be covered in a good layer.
Place a heaping tbsp of filling at one end of each of the zucchini slices. If using smaller zucchini slices lay two side by side, slightly overlapping them.
Roll the slice into a tight coil and nestle it seam-side down on top of the tomato sauce. Drizzle lightly with olive oil. Season lightly with salt and pepper. Transfer to the oven and bake until the sauce has reduced slightly and is bubbling, and the rolls are lightly golden on top, about 15 minutes.
At this point take the pan from the oven and top evenly with the torn or shredded fresh mozzarella, and return to the oven for another 10-15 minutes.
If the rolls are not golden and the cheese hasn't started to bubble on top: Broil the involtini for about 3-5 minutes keeping a close watch the entire time. Remove pan from the oven and allow to cool slightly before serving.
Finish with chopped parsley or basil.
The cooking times will vary depending on how thinly you slice your zucchini. Know that the timing is forgiving, however—you just need the zucchini to be pliable so that they can roll into a coil. You want enough zucchini slices to create 12-15 roll ups.
The sausage in the marinara is totally optional if you are desiring to make a meatless meal. Feel free to make a double batch and store the remainder in the freezer for future involtini opportunities.
To make at the same time, prepare the marinara on the stove top, the zucchini in the oven and then the spinach ricotta filling while the marinara sauce is simmering.