1. Preheat oven to 325 degrees Fahrenheit (160 degrees Celsius).
2. Slice zucchini lengthwise into 1/4-inch (1/2-centimeter) thick slices. (I used a mandoline to get perfectly even slices.) Select the 15 longest slices and place onto two baking sheets in a single layer. Roast for 15 minutes, just until soft enough to roll, switching the pans midway through baking. Remove from oven and allow to cool.
3. While the zucchini is roasting, prepare the filling by placing the ricotta cheese, spinach, two cups mozzarella cheese, 1/2 cup Parmesan cheese, beaten egg, parsley, scallions, and salt in a medium bowl. Mix well to combine.
4. Working on a flat surface, spread a spoonful of filling over the length of each zucchini slice and roll up gently. Divide the filling evenly over all 15 slices.