Thaw the spinach on the countertop, or, for a quicker method, place it in a microwave-safe bowl and zap until defrosted, about five minutes. Squeeze out as much water as you can.
Heat the olive oil and add the spinach along with salt and ground black pepper. Saute until the spinach is quite dry and cooked. Transfer to a large bowl.
Drain the cashews and blend them with one cup of water until you have a very smooth paste. Add to the bowl with the spinach.
Add the all-purpose flour, nutritional yeast, and nutmeg to the bowl along with more salt if needed. Mix well.
Place a large pot of water to boil.
Place the flour for dusting on a plate and drop a spoonful of the spinach dumpling batter on the flour. Turn to coat with the flour then, using your fingers, gently shape into a ball. Place on a floured plate until you shape the rest of the dumplings. I got 24 dumplings, about an inch in diameter.
When the water has boiled, gently drop the dumplings into the water, one at a time. Cook for about 7-8 minutes, but don't stir them as they cook because you don't want to break them. As the dumplings cook, they will rise to the surface.
Using a slotted spoon, fish the dumplings out of the water and place them on a plate covered with paper towels.
To serve, place some marinara on a plate, and top with the dumplings.