Place the puff pastry slices next to each other and let them thaw slightly. In the meantime peel the onions and cut them into rings. Crumble the feta cheese and chop the olives. Strip rosemary needles from the branch, chop finely.
Mix prepared ingredients and sour cream. Season with a little salt and chili. Rinse the tartlet cups (approx. 100 ml content) with cold water. Put 1 slice of puff pastry in each. Spread the filling in it, beat the protruding puff pastry corners inwards.
Spread with whisked egg yolk, sprinkle with sesame seeds. Bake in the preheated oven (electric oven: 225 °C/ convection oven: 200 °C/ gas: level 4) on the bottom rack for 15-20 minutes. Garnish with chillies and rosemary.