Crumble the yeast and dissolve in 4 tablespoons of warm water. Knead with flour, salt, buttermilk and 2 tablespoons of oil, first with the dough hook of the hand mixer, then with hands to a smooth dough. Form into a ball and cover in a warm place for 2 hours.
Peel the asparagus, cut off the ends. Use an asparagus peeler to cut lengthwise into thin slices and cover with a damp kitchen towel. Pluck rosemary needles from twigs. Slice cheese into fine shavings. Preheat baking tray in the oven at 250 degrees (convection oven not recommended).
Knead dough with hands and divide into 4 portions. Roll out each portion thinly on baking paper. Let rest for 10 minutes, then roll out even thinner.
Spread the pastry with crème fraîche, cover asparagus slices, sprinkle rosemary and season with pepper and sea salt. Bake one after the other on a hot baking tray in the middle of the oven for 7-9 minutes until golden brown.
Place 3 slices of ham and 1 tablespoon of cheese on each. Sprinkle with ½ tbsp. oil and serve immediately.