Cut the lasagna sheet in half, and cut each strip in half again. Place the pasta sheets into a bowl, and pour very hot water in it, until it covers all the pieces. I used boiled hot water from my electric kettle. Set the pasta pieces aside.
Chop the spinach and place it in a microwave-safe bowl. Cover the top with plastic wrap and poke a few holes in it for ventilation. Microwave for 1 minute. Take out the spinach and let it rest.
Grate the mozzarella while you wait for the spinach to cool.
Mix the spinach with the ricotta, pepper, granulated garlic, and salt. Set mixture aside.
Pour 2 tablespoons of pasta sauce at the bottom of the mug. Top with a piece of softened pasta sheet. Add some spinach mixture, 2 tablespoons of mozzarella, and a pasta sheet. Continue building the lasagna, ending with a layer of pasta sheet at the top. Sprinkle mozzarella over the pasta.
Microwave everything for 1 minute 30 seconds. Check to make sure that the mozzarella is melted. If it isn't, continue microwaving at 15-second intervals until the cheese is fully melted.