Preheat the oven to 400° F (200°C)
Start by making the almond ricotta: steam cauliflower florets for 8-10 min until fork tender. Lift out of steamer using a slotted spoon (leave steamer assembled) into a food processor and let cool slightly.
If you want the ricotta to look white, slip the almonds out of their skins while the cauliflower cooks and place the naked almonds into the food processor along with the cauliflower. It's extra work...but it adds to the look. If you don't mind a speckled look to your ricotta, leave the skins on - it's healthier that way!
Then, add the garlic and salt and pulse until mixture is finely chopped. Scrape the sides of the processor and start drizzling in the lemon juice as you process to form a thick paste. Set ricotta aside while you prepare the rest of the lasagna.
Prep the Celeriac: Cut celeriac into two halves, or quarters if need be to fit your mandoline. Using a mandolin on the thinnest setting, finely slice the celeriac and set aside. Using a mandolin is important here as if the celeriac isn't paper thin, it will take forever to cook! If you don't have a mandoline, you could parcook the celeriac in boiling water until fork tender.
In the same steamer you used for the cauliflower, steam the spinach until wilted, about a minute. Let cool and carefully squeeze water out.
Next, cook the mushrooms: in a large skillet over medium high, heat the olive oil and add the mushrooms and cook, stirring occasionally until they get a nice sear and start to release their water. Try not to fuss with them too much so they can brown.
Once they release their water, add the garlic and stir constantly until water evaporates. Turn off heat and add thyme, salt and pepper. Set aside.
Now, assemble the lasagna. In a 9 x 12 casserole dish, spread on 1/3 of the pasta sauce. Next, add a layer of celeriac noodles, overlapping them slightly.
Dollop on half of the ricotta mixture, pat gently to evenly distribute. Layer on half of the mushrooms and spinach.
Next, follow the same pattern: celeriac, remaining half of the ricotta, remaining half of the spinach and mushrooms and top with 1/3 of the sauce. Add a third layer of celeriac and spread on the last 1/3 of the sauce.
Cover the dish with foil and bake at 400 for 45-50 minutes, checking the celeriac for tenderness. I like it a bit al dente.
Remove foil and bake uncovered for 5-10 more minutes to get the sauce bubbling on top. Let sit for ten minutes before garnishing with pine nuts and basil and serve.