Add the onion and garlic and sweat over a medium heat until translucent. This can take 8-10 minutes.
Add the courgettes, peppers, sage and a pinch of salt and pepper and fry for a further 5 minutes.
Add the tomatoes, basil and bay leaf.
Stir, bring up to a simmer, then leave bubbling away for about 15 minutes, stirring occasionally until it has reduced.
Turn off the heat, season to taste and set aside.
Prepare the potatoes
While the tomato sauce is simmering, you can get on with the other elements.
Cut any of your large potatoes in half so that they’re all of a similar size.
Place in a pan and cover with water. Bring to the boil and simmer for 5 minutes until just becoming tender.
Tip into a colander to drain and leave until cool enough to handle.
Cut into thin 2-3mm slices and place on a plate, drizzling sparingly with a little olive oil as you go so that they don't all stick together.
Make the white sauce
Put the milk, flour and butter in a non-stick pan.
Whisk over a gentle until it begins to thicken. Taste to make sure the flour has 'cooked out' , meaning you can't taste it.
Add the grated cheese, parsley and pinch of salt and pepper.
Stir through until melted, then take off the heat and season further, if necessary.
Assemble the lasagne
Grease a large lasagne tray or casserole dish.
Spread a half of the tomato sauce, then cover with a single, slightly overlapping layer of potato. Repeat with the other half of the sauce and another layer of potato.
Pour the cheese sauce on top, spreading to the edges.
Sprinkle the grated cheese on top of the lasagne, followed by the parsley and a good pinch of pepper.
Place in the oven at 200C (180C fan/400F) and bake for 20 minutes. Check half way and if the top is looking like it's browning too soon, just cover the dish with a hat of silver foil and return to the oven to finish baking.
Push a sharp knife into the centre of your lasagne to make sure the potatoes are cooked through - there should be no resistance.
Your lasagne will firm up as it cools, so if you want to serve neat slices allow it to cool for 15 minutes before cutting and plating up.