Finely dice the onion. Heat the oil in a deep saucepan and sauté the onion for 3-4 minutes on medium heat.
In the meantime, finely dice the bell pepper and press the garlic. Add them to the pan and sauté for another 4 minutes.
Drain the lima beans and add to pan, together with the tomato polpa. Fill the tomato can with water and add that too. Add the italian herbs, salt and sugar, turn up the heat and give a good stir.
Break the lasagna sheets into smaller pieces (about quarters). Be careful the lasagna shrapnel doesn't hurt you! (: Fill the pan with the pieces and make sure they are all covered with sauce. Turn the heat to medium-low, so the sauce can softly bubble. Cover the pan. In total, the lasagna should be on the cooking softly for 25-30 minutes.
Now make the béchamel: Take a small saucepan and add the vegan butter on medium heat. When it's melted, add the flour and mix it all together. Heat for 1 minute. Then, add the plant milk at once, and keep on whisking. Just keep whisking until the sauce comes together. Then add the salt and nutritional yeast.
Pour the béchamel sauce over the top of the lasagna. Put the lid back on and wait for the cooking time to elapse.
The lasagna is done when the pasta sheets have fully softened. Finish off with black pepper and parsley.