Preheat oven to 190 C / Gas 5. Grease an oven proof casserole dish with the olive oil.
Place 1/3 of courgette slices into the casserole dish and top with 1/3 of the onion. Season with salt, pepper and herbs and cover with 1/2 the Gouda slices. Add another layer of courgettes, onion, seasoning and top with havarti slices. Place the remaining courgettes, onions, seasoning and Gouda cheese on top. Cover with aluminium foil.
Bake for 35 minutes in the preheated oven. Uncover and bake for another 15 to 20 minutes, until golden brown on top.