Preheat oven to 425° F (218° C).
Cut top and bottom off each eggplant and cut into 1/8” slices lengthwise (easiest using a mandolin, makes about 18 slices).
Sprinkle eggplant slices with salt and let sit overlapping in a colander for at least 30 minutes, preferably 1 hour. Rinse with cold water to remove salt and pat dry with paper towels or a kitchen towel.
Spray a baking sheet with cooking spray and lay eggplant slices out in a single layer (may need two baking sheets or to roast in two batches). Sprinkle lightly with salt and pepper and bake in oven for about 10 minutes. Remove from oven and let sit until cool enough to handle. Reduce heat to 375° F.
Meanwhile, heat oil in a skillet over medium heat. Add onion and cook for 5-6 minutes until onion is translucent and soft. Add garlic and cook a minute longer. Add spinach and cook down for 1-2 minutes until just wilted. Remove from heat and set aside.
In a small bowl, whisk the egg with a fork. Add ricotta, nutmeg, and basil and stir to mix.
Add ricotta mixture to cooked down spinach. Season to taste with salt and pepper (about ½ tsp. salt and ¼ tsp. pepper).
Spoon about 1 cup marinara sauce into 9x13” baking dish.
Spoon about 2 Tbsp. of ricotta spinach filling into the large end of an eggplant slice. Roll and place seam-side down into sauced baking dish. Repeat with remaining eggplant slices, making 3 rows of six.
Spoon remaining marinara sauce over eggplant rolls. Sprinkle with mozzarella cheese and bake for about 25 minutes until cheese is melted and filling is starting to ooze out the sides of each roll. Remove from oven and sprinkle with fresh parsley or additional chopped basil before serving if desired.