Preheat the oven to 350°F. Line a rimmed -baking sheet with parchment paper.
Spread the walnut halves in a single layer on a small rimmed baking sheet and toast until light golden and aromatic, 6 to 8 minutes. Pour onto a cutting board, let cool, and chop coarsely. Leave the oven on.
In a medium bowl, sift together the flour, cinnamon, cloves, baking soda, and salt. Set aside. In a mixer fitted with the beater, combine the sugar, butter, egg, molasses, and 2 tablespoons water and beat on medium speed until smooth, about 2 minutes. Add the flour mixture to the sugar-butter mixture and beat on low speed until well combined. Using a rubber spatula, fold in the walnuts and raisins.
Divide the dough in half. On a lightly floured work surface, roll half of the dough into a log 1 inch in diameter. Transfer the log to the prepared baking sheet and flatten it until it is 11/2 to 13/4 inches wide. Repeat with the remaining dough, spacing the logs about 3 inches apart.
Brush the top of each log lightly with the reserved whisked egg, then bake until light golden on the edges and slightly soft in the center, about 17 minutes. Let the logs cool on the pan on a cooling rack for about 5 minutes.
Transfer the logs to a cutting board, and while still warm, cut each log on the diagonal into slices 1 inch wide. Return the cookies to the rack to cool completely. Store in an airtight container at room temperature for up to 5 days.