Lets start with the spinach. I would normally steam it, but in this case I just blanched them in boiling salted water for about 10 seconds or until just wilted.
Transfer to a cold water bath or just sprinkle cold water to stop it from cooking further and let it cool. Takes about a minute.
Now squeeze out the water as much as possible.
Chop it coarsely and set aside.
In a pan, melt 1 tbsp butter and add the mushrooms.
Cook them until slightly browned. Set aside
In the same pan, melt 2 tbsp butter in medium heat
until its starts foaming.
Add the onion and garlic paste and cook whisking it until the onions are soft.
Next, add in the flour and cook - keep whisking it to avoid the butter from browning. We do this in order to toast the flour (to avoid the raw grainy taste later on)
Gradually add in the milk
and bring the mixture to a boil over med-high heat. Keep whisking it. Add the bay leaf, nutmeg, salt and pepper and simmer over low heat for about 10 minutes. Whisk it on and off.
When you find the mixture is thick and saucy, add in the Parmesan cheese and whisk until mixed. Remove the bay leaf.
Set aside abt 1/4 cup of the sauce and in the remaining sauce, whisk in the spinach until well combined. You would have around 2 cups of the spinach-bechamel sauce
While this is happening, in another pan, cook pasta according to the manufacturer's instructions.
In another bowl, whisk the ricotta cheese along with salt until smooth. Set aside.
Preheat Oven to 425F. Grease your baking dish with some butter. I used a 6 cup (1.5 L) 8X6X2" dish for my lasagna, but you can use whatever you have. Layer the bottom with some bechamel sauce (abt 1/4 cup that you set aside).
Now place 3 noodles on top of it, side by side. Since I did not have much of a space, I overlapped my noodles.
Note: From here on you can layer it as your fancy strikes. Just make sure to have some sauce and cheese leftover for the last layer to cover the noodles.
Spread about 1/2 cup of the spinach mixture.
Sprinkle Parmesan over the spinach mixture.
Layer that again with 3 more noodles.
Layer that with 1/2 cup spinach mixture and 1/2 cup of Mozzarella cheese.
Layer the ricotta cheese mixture over it and spread until it covers the layer,
followed by mushrooms.
I cut the remaining 1 lasagna sheet into half and covered the middle and enclosed the overhanging noodles (that I layered before) on the ends. Now you know why the odd number 7.
Cover with remaining spinach mixture completely enclosing the pasta
and toss in the mozzarella cheese as last layer.
Cover the top with foil (grease the inner side of the foil with some oil so that it doesn't stick to the cheese)
and bake for 10-15 min or until it starts bubbling. Remove the foil
and now Broil the lasagna for about 5 minutes or until the cheese is browned in spots. Let it cool for about 10 minutes