Toast the walnuts in a dry 12-inch skillet over medium heat for 4 to 5 minutes, stirring, until fragrant and lightly browned. Let cool, then chop coarsely and set aside.

Zest the lemon and set the zest aside. Halve the zested lemon and rub the cut side over a marble slab or a sheet pan lined with parchment paper, to keep the caramel from sticking.

Combine the sugar, honey, and water in a 3-quart / 2.8 L heavy-bottomed saucepan. Set over medium heat and stir gently until the sugar dissolves.

Stop stirring once the sugar dissolves. Let the mixture bubble, swirling the pan occasionally, until it turns deep amber and reaches 150 C / 300 F on a candy thermometer, about 8 to 10 minutes.

Remove the pan from the heat immediately. Stir in the toasted walnuts, lemon zest, and salt, working quickly so the caramel doesn't seize.

Pour the hot mixture onto the prepared marble slab or sheet pan. Spread it to about 1 cm / 0.4 inch thickness with an oiled offset spatula.

Let the brittle firm up for 3 to 4 minutes, until it's warm but no longer spreads. Score it into rough rectangles or diamonds with an oiled knife.

Let cool fully at room temperature for 30 minutes, until hard throughout. Break along the scored lines into pieces.

Store the pieces in an airtight tin at room temperature.