Put the serrano ham slices on an oven tray and cook at 180C por 5-10min or until crispy. Chop and reserve.
Remove the top dark green part of the leeks and keep the light green bottom part. Cut in half lengthwise and wash moving the leaves to release all the dirt that's between them. Slice.
Cook with a bit of olive oil on medium-low heat along with the leaves from the rosemary sprigs until the leeks are nice and soft. From them cook them a bit further until they start to get golden.
Remove the stalk from the mushroom and cut in 4.
Cook in the same pan as the leeks with a bit more olive oil and this time on medium-high heat. Don't fill the pan too much but add enough to cover the base of the pan. This is so that they can get golden. Don't move them around until they have become golden on one side and them flip and repeat. Add salt at the end and let them cook until they don't release any more liquid. Reserve and cook the rest of the mushrooms the same way.
Mix the leeks and mushrooms in an oven-safe dish and cover with a thin layer of breadcrumbs.
Spread the serrano ham evenly over the surface.
Make another thin layer of breadcrumbs.
Put small cold butter cubes on top and take to a preheated oven at 180C for 20min or until it's golden on top.