For the cake: Preheat the oven to 350 degrees F. Grease and flour a tube pan and set aside.
Using an electric mixer, beat the butter and the granulated sugar together until creamy. Add the eggs, one at a time, beating until just blended after each addition. Sift together the flour and the baking soda. Add the flour mixture to the butter mixture, alternating with the sour cream, beginning and ending with the flour mixture. Add the vanilla, lemon zest, poppy seeds, and thyme leaves (if desired) and beat until fully combined.
Pour the batter into the tube pan and bake for 60 to 75 minutes, or until a toothpick inserted near the center comes out clean. Remove from the oven and allow to cool in the pan for 5 minutes, then turn out onto a rack to cool completely, about 20 minutes.
For the glaze: While the cake is still warm, in a small bowl, mix together the lemon juice and confectioners' sugar. Pierce the top of the cake with a fork to create holes, then pour the mixture over the cake on the rack, allowing the glaze to soak in. Garnish with thyme leaves if desired and Candied Lemons. This cake is great served warm and will keep refrigerated for up to 2 weeks.
Candied Lemon Slices:
Bring a medium saucepot with water to a boil and remove from the heat. Place the lemon slices in the hot water and let sit 1 to 2 minutes until the slices soften. Drain the slices from the water and place into an ice bath; leave in the ice water until the sugar syrup is ready.
Put 1 3/4 cups of the granulated sugar and 1 3/4 cups water in a saute pan and bring to a boil. Reduce the heat to medium low. Place the slices directly into the sugar syrup and cook on medium low for 1 hour. Remove the slices to a rack on a baking sheet to dry.
Toss the slices in the remaining 3/4 cup granulated sugar to coat. Store in an airtight container until using.