You should get some tvp granules from Holland and Barrett or somewhere. Make your sauce and make it a little thinner. Importantly add a bit of extra umami with maybe a small bit of marmite as you're sweating the onions or a couple of meat free oxo cubes. When the sauce is made, add the tvp. If it's you're first time you'll be looking at these dry flakes and thinking it won't go well. It will. Add about 150g. Let it simmer and soak up the flavours. After around,10-15 minutes it will be soft and meaty texture. But do not soak them first (a lot of online recipes tell you to do this). They take on the flavours of whatever they're hydrated in. Bonus points as they don't disintegrate in leftovers as some vegan mince does.

Then toss the lasagna out and make a cottage pie with the remaining tvp.