Combine the sugar, honey, water, cinnamon stick, and lemon peel in a 2-quart / 1.9 L saucepan. Bring to a boil over medium heat, then simmer for 8 to 10 minutes until the syrup coats the back of a spoon. Stir in the lemon juice, remove the cinnamon stick and peel, and let the syrup cool to room temperature while you work.

Heat the oven to 175 C / 350 F with a rack in the middle, and line a 33 x 46 cm / 13 x 18-inch rimmed sheet pan with parchment. Pulse the walnuts, 25 g sugar, ground cinnamon, cardamom, and salt in a food processor 6 to 8 times, until the nuts look like pea-size crumbs. Don't run it continuously or you'll get paste.

Unroll the thawed phyllo and stack the 15 sheets. Cut the stack in half crosswise, then cut each half in half again, to make 60 rectangles about 11.5 x 18 cm / 4.5 x 7 inch. Cover them with a damp kitchen towel, then melt the butter.

Lay one rectangle on the counter with a short side facing you and brush it lightly with butter. Top with a second rectangle, brush, then add a third. Spoon a heaping tablespoon (about 14 g) of filling in a line 2 cm from the edge nearest you, leaving 1 cm bare at each side. Fold the side edges in, roll up tightly away from you, and set the roll seam-side down on the pan. Repeat to make 20 rolls, placing them 1 cm apart and keeping the unused phyllo covered.

Brush the tops and sides of the rolls with the remaining butter. Bake for 25 to 30 minutes, rotating the pan once halfway, until deep golden brown and crisp.

Pour the room-temperature syrup slowly over the hot rolls the moment they leave the oven, listening for the sizzle, so each roll is coated. Let the rolls rest uncovered at room temperature for 60 minutes, until the syrup is absorbed and the surfaces turn glossy.