Put the spinach into a large bowl and cover with boiling water. Leave to sit for 10 minutes.
While this is soaking, mix the tomato layer ingredients together in a jug and set aside.
Drain the spinach using a sieve or colander, squeeze out as much water as you can (once it's a little cooler, I use clean hands to squish it and do this).
Now chop the drained spinach finely and mix it with the other ingredients of the spinach layer.
In your slow cooker, layer a third of the spinach mixture, followed by a layer of lasagne sheets, a third of the tomato mixture and a third of the mozzarella.
Repeat twice more until everything is used up.
Lay a clean tea towel over the top of the slow cooker and sit the lid on top (this helps to absorb water so that the lasagne doesn't get too wet).
Cook on HIGH for 2-3 hours or LOW for 5-6 hours until the pasta is just cooked through.