Prepare the Cake: Stir together warm water (105°F to 115°F), yeast, and 2 teaspoons of the sugar in a small bowl; set in a warm place until foamy, about 5 minutes.

Beat salt, nutmeg, zest, 4 cups of the flour, and remaining 1/2 cup sugar in a stand mixer fitted with a paddle attachment on medium-high speed until combined, about 30 seconds. Add egg yolks, warm milk, melted butter, and yeast mixture; beat on medium-high speed until smooth, about 1 minute.

Turn dough out onto a lightly floured work surface; knead in up to remaining 1 cup flour, 1/3 cup at a time, until dough is no longer sticky. Continue kneading until dough is smooth and elastic, about 10 minutes.

Place dough in a bowl coated with cooking spray, turning to grease top. Cover and let rise in a warm place (85°F) free from drafts until doubled in bulk, about 1 1/2 hours.

Punch down dough, and place on a lightly floured work surface. (If desired, sprinkle with candied citron; knead until citron is evenly distributed.) Shape dough into a cylinder 30 inches long.

Place dough cylinder on a buttered baking sheet; shape into a ring, pinching ends together to seal. Press pecan half gently into dough ring from underside of dough so it is completely hidden inside the dough. Cover and let rise in a warm place until doubled in bulk, about 45 minutes. Meanwhile, preheat oven to 350°F.

Bake Cake in preheated oven until golden brown, 25 to 30 minutes. Let cool on baking sheet 30 minutes.

Prepare the Glaze: Stir together powdered sugar, lemon juice, and water in a small bowl until smooth.

Drizzle Cake with Glaze; sprinkle with sanding sugar, alternating colors.