I've made this meal a million times, it's originally from Linda McCartney's home cooking book, but over the years I've taken a bit out and added a bit in.
I make it from sight so I'm not sure of measurement etc.
Gently fry a chopped red onion in a little olive oil.
Add a crushed garlic clove.
Chopped celery (sometimes)
When onion is soft add two tins of chopped tomatoes (one if doing vegetable version)
Now the questionable measuring...
Some tomato purée I'd say maybe half a tube or two tablespoons!!!
Some passatta if I have some.
A little bit of veg stock cube or marigold bouillon.
Always a teaspoon of sugar to take away the acidity of the toms.
Quorn mince can be added or carrot courgette, red pepper, aubergine and mushroom for a vegetable version, omit one can if toms as I said.
I'm a bit lazy as I don't make my own white sauce!
Ladle a layer of the tomato sauce into a large rectangle dish.
Then a layer of lasagne fresh, dried whatever you like.
Repeat with sauce and lasagne.
Then a jar of white sauce, I've tried most now and have to say Aldi's is as good as Ragu or Dolmio.
I put it in the oven now for about 15 minutes then take it out sprinkle mozzarella cheese on top and return it to the oven for another 15 mins.