Pre-heat oven to 200 Celsius, and start boiling the lasagna noodles in a huge pot in slightly salted water.
Sautee the onion and garlic cloves in the olive oil until soft. Chop all the mushrooms very finely into 1 cm cubed pieces. They will become your mince substitute. Add all the mushrooms and continue to sauté until the mushrooms are soft and have reduced in size. Add your pasta sauce or passata and salt, pepper, pinch of sugar (trust me) and the Italian seasoning. Taste the sauce and add more of any ingredient you personally feel it needs. Set sauce aside.
Next, make the white sauce. On medium heat, melt down the margarine and slowly add the flour to the melted margarine with a whisk until it forms a dough in the pan. Still whisking, add the soymilk and your garlic puree, along with more salt & pepper. Sauce should thicken, and once its done you can take it off the heat to prevent burning.
Now its time to assemble: Start with a layer of the lasagna noodles, then spoon the mushroom bolognaise sauce over it. keep going until you reach about 8 cm from the top. add you final layer of noodles, then pour the creamy white sauce on top. Top with some grated vegan cheese if you have it, but its perfectly fine on its own. Give it a final crack of black pepper and pop it into the oven for about an hour.