Put the pastry on a lightly floured kitchen bench and roll it out into a rectangle measuring 36 × 45 cm (14 1⁄4 × 17 3⁄4 in) and 4 mm (1⁄8 in) thick. Lay the pastry so that one long side is parallel with the edge of the bench.
On one short side, mark four notches along the edge of the pastry, starting from the bottom: at 8.5 cm (3 1⁄4 in), 17 cm (6 3⁄4 in), 19 cm (7 1⁄2 in) and 27.5 cm (11 in). You will use these notches as a guide for filling and folding the palmiers.
Spread the mustard evenly over the pastry, avoiding the 2 cm (3⁄4 in) strip between the middle two notches. Lay the ham over the pastry in two strips – one strip between the first and second notches and another strip between the third and fourth notches. Use half the ham for each strip. Lay the cheese directly over the ham, again using half for each strip.
Lift up the bottom section of the pastry nearest you and fold it over the ham and cheese to meet the notch at 17 cm (6 3⁄4 in), then lightly brush with egg wash. Fold the top section down to meet the notch at 19 cm (7 1⁄2 in) and repeat the brushing. Both strips of filling should now be covered in pastry, leaving a 2 cm (3⁄4 in) gap in the middle. Next, fold the top half down over the bottom half, using the 2 cm (3⁄4 in) gap as a hinge.
Brush the exposed pastry with egg wash and sprinkle generously with sesame seeds. Gently turn the roll over and repeat on the other side, so the entire exposed surface of the pastry is well coated with seeds. Put the roll on a tray and refrigerate for 30 minutes to firm up the pastry – it will be easier to cut when chilled.
Line a large tray with baking paper. Use a sharp knife to trim the edges of the pastry roll, then cut 2 cm (3⁄4 in) slices through the cross- section of the pastry. Transfer the slices to the lined tray, taking care they don’t unravel. Chill for at least 30 minutes before baking. At this stage, you can wrap and freeze them until needed.
The Taste Edit - Ham and Chees Palmier https://thetasteedit.com/ham-and-cheese-palmiers/