Peel and finely chop onion and garlic.
Rinse the mushrooms and chop them into thin slices.
Fry the onion and garlic in a big pan with a couple of tbsp of olive oil on a medium heat until soft.
Add the mushrooms and balsamic vinegar and let it fry until the mushrooms softens.
Add rinsed spinach and let it get mushy. If there isn't enough room on the pan, you can do this step on another pan and then add the spinach to the mushrooms after.
Taste with salt and pepper.
Make the bechamel by melting the butter in a small pot. Whisk in the flour and fry for a minut or two while whisking.
Add the pea milk a little at a time while whisking, until it thickens and then add some more pea milk. Keep going until all of the pea milk is used and the bechamel is thick.
Add finely grated lemon zest, half the grated cheese and half of the tarragon, finely chopped. Taste with salt and pepper.
Layer the lasagna in a casserole: first a bit of bechamel and mushroom filling and then fresh lasagna sheets, ½ of the remaining mushrooms filling, ⅓ of the remaining bechamel and repeat this one more time.
Top with the last layer of lasagna sheet (three layers in total) and the last bechamel - make sure it covers all the sheets. Sprinkle with the last cheese and tarragon.
Bake at 180 degrees celcius fan for about 25 minutter.