Halve the potatoes and cook them in boiling lightly salted water until tender, then drain and leave until cool enough to touch. Meanwhile, melt 15g of the butter in a saucepan. Add the bacon and sizzle until golden. Remove with a slotted spoon, then add the onion and pears. Cook until pale golden, then tip the bacon back into the saucepan.
Remove the kale leaves from their coarse ribs and blanch the leaves for 2 mins in boiling water. Drain and rinse in cold water. Chop the leaves and add to the bacon and onions. Pour in 2 tbsp water and some pepper, cover the pan and cook for about 15 mins, making sure the pan doesn’t become dry. Heat oven to 200C/180C fan/gas 6.
Peel the potatoes and put them back in the pan in which they were cooked. Add the remaining butter and seasoning, then mash together.
If the bacon mixture is too ‘wet’, then remove the lid, turn up the heat and boil off the excess moisture. Put the kale and bacon mixture into a gratin dish and spread the mash on top. Sprinkle over the cheese, if using, then bake in the oven for about 20 mins, or until the top is golden – you can crisp it up under the grill for 5 mins if you need to. Leave to rest for a few minutes before serving.