Preheat oven to 180C.
In a bowl, mix the ricotta cheese with the pesto and 100ml of water. Season with salt and pepper.
In an ovenproof dish, start with a layer of courgette, peas and butter beans, spooning some of the stock/liquid with them as you go. You’ll do three layers, so use approximately a third of each.
Spoon over some of the ricotta pesto sauce and spread it evenly.
Top with a layer of pasta, breaking any edges if you need to to make them fit.
Then top with a thin layer of the pesto sauce too (the pasta needs moisture both sides to cook) and a third of the mozzarella. Season with salt and pepper.
Repeat again twice more, seasoning each layer as you go.
On the top, cover with the remaining pesto sauce but keep the remaining mozzarella aside.
Cover the dish with tin foil (you might need two pieces) so it’s sealed - this will trap the moisture in.
Cook for 45 minutes with the foil on.
After 45 minutes, remove the foil, turn the heat up to 200C and add the remaining mozzarella cheese (during this step, if any corners of the lasagne are sticking up so they are not in sauce, push them down).
Return to the oven for a further 15 minutes until the top is golden brown.
Remove from the oven and let it sit for a 10 minutes before serving.
Slice and dig in.
