Cook lasagna noodles in well-salted boiling water for 9 minutes.
Meanwhile, place frozen spinach in a fine-mesh sieve and run hot water over it until soft. Squeeze out as much water as you can and transfer it to a medium bowl. Stir in ricotta, salt and garlic. Set aside.
Drain the lasagna noodles (just tip the pot into the sink carefully using the lid of the pot to prevent pasta from escaping if you want to avoid dirtying a colander). Add marinara sauce to drained pasta in the pot and heat through on low heat.
Dollop ricotta mixture by spoonfuls into the hot sauce-coated pasta, lifting noodles to tuck some under the top layer. Tear apart the fresh mozzarella and scatter on top. Close pot lid and turn off the heat to melt the cheese, about 3 minutes.