In a food processor, combine the almonds and hazelnuts. Pulse until finely chopped. Transfer to a large bowl and add the flour, spices, baking powder, and salt. Whisk until combined.

Add ½ cup of the flour mixture and the candied orange and lemon peel to the food processor. Pulse until finely chopped. Stir the chopped citrus peel into the bowl with the flour mixture.

In a large mixing bowl or the bowl of a stand mixer fitted with a paddle attachment, beat the brown sugar, honey, and egg together on medium-high speed until thickened and fluffy, about 1 minute. Add the flour mixture and mix on low speed until just combined. Scrape down the dough, cover, and let rest at room temperature for at least 1 hour or up to 8 hours.

Preheat the oven to 350F. Line two rimmed baking sheets with parchment paper.

Scoop the dough into 2-tablespoon-sized balls and place on the parchment paper about 2 inches apart. Flatten them into 2½-inch wide discs using the back of a wet spoon.

Bake one sheet at a time for 20 minutes or until the edges start to brown and the centers are still soft but appear dry. Let cool for a few minutes on the pan, then transfer to a wire cooling rack.

While the cookies are still warm, make the glaze by whisking together the powdered sugar, warm water or rum, and lemon juice. Brush the glaze over the warm cookies. Decorate with blanched or slivered almonds, if desired.