Preheat the oven to 210°C (410°F). Spread half of the crushed tomatoes/pasta sauce in the bottom of a shallow baking dish and set aside.
Slice the zucchini lengthwise into wide thin ribbons using a mandoline, vegetable peeler, cheese slicer, or a sharp knife; set the ribbons aside.
Place the thawed spinach in a bowl and squeeze out as much liquid as possible, then discard the liquid.
In the bowl with the spinach, combine the ricotta, eggs, crushed garlic (or garlic powder), and 1/2 cup of the mozzarella. Stir until well combined, then season with salt to taste.
Lay two zucchini ribbons overlapping slightly, spoon 1–2 tablespoons of the ricotta-spinach mixture onto the narrow end, then roll up tightly to form a cannelloni. Place each roll seam-side up in the prepared baking dish.
Repeat with remaining zucchini and filling. Pour the remaining sauce over the rolls and sprinkle with the remaining mozzarella.
Bake for 20–30 minutes, until the cheese is golden and bubbling and the zucchini is tender when pierced with a fork.
