This is assuming you're using the ready-bake noodles, if not, cook the noodles before assembling per package instructions
Preheat the oven to 375 Farenheit
Using olive oil or cooking spray, lubricate your 9x13 baking dish
Add oil to your frying pan and heat for a minute or two over medium heat
Add the onion, letting that cook for 3-4 minutes before adding the garlic and cooking for another minute
Add the sliced mushrooms, dried thyme, salt and pepper and cook the mixture until the mushrooms are soft and browning. Most of the liquid released from the mushrooms should be evaporated when you're done
Add the chopped spinach leaves to the pan, stirring and cooking until they wilt, usually only 2-3 minutes
To assemble: Ladle some marinara on the bottom of the pan and add your first layer of noodles
Put half of the ricotta cheese on top of the noodles and spread to cover
Add a layer of the mushroom mixture and top with about 1/3 of the mozzarella and parmesan
Repeat these layers of noodles, ricotta, mushrooms, cheese until your pan is full. Be sure to leave just enough room to top the whole thing with as much marinara as you can fit.
Cover this with foil and bake for 25 minutes. Remove foil and continue to bake for another 10-15 minutes or the cheese is nice and bubbly taking on some browning
If you want the best results, let this sit for about 10 minutes before slicing in so everything has a chance to meld together OUT of the oven, but otherwise serve it up!