Combine the water, sugar, salt, cinnamon stick, and orange zest in a 2-quart / 1.9 L saucepan. Bring to a boil over medium-high heat, then lower the heat and simmer for 5 minutes.

Remove the cinnamon stick and orange zest strip. Keep the syrup hot over low heat while you toast the semolina.

Melt the butter in a 26 cm / 10-inch heavy-bottomed pot over medium heat.

Add the semolina and stir constantly for 8 to 10 minutes, until it turns pale gold and smells toasted and nutty. Stir in half the sliced almonds during the last 2 minutes so they toast lightly too.

Reduce the heat to low. Pour the hot syrup into the semolina in a slow, steady stream while stirring constantly with a long wooden spoon. The mixture will bubble and steam, so pour carefully and keep your face back.

Keep stirring for 3 to 5 minutes, until the pudding thickens and starts to pull away from the sides of the pot. If lumps form, that just means the syrup went in too fast, keep whisking hard and they'll smooth out before it's done.

Cover the pot and remove it from the heat. Let it rest for 10 minutes so the semolina finishes absorbing the liquid and turns fully soft.

Spoon the warm halva into a 20 cm / 8-inch bundt pan or mold, pressing down firmly to remove air pockets. Let it sit for 15 minutes.

Run a knife around the edge and invert the pudding onto a serving plate. Dust with ground cinnamon and scatter the remaining almonds over the top.

Serve warm or at room temperature, sliced into wedges.