Preheat oven to 350°. Spray 2 (8×5-inch) loaf pans with baking spray with flour.

In a large bowl, beat butter and sugar with a mixer at medium speed until creamy, about 6 minutes, stopping to scrape down sides of bowl. Add poppy seeds and thyme, beating until combined. Add eggs, one at a time, beating well after each addition.

In a medium bowl, sift together flour, baking powder, and salt twice. In a small bowl, whisk together buttermilk, lemon zest and juice, and vanilla. Gradually add flour mixture to butter mixture alternately with buttermilk mixture, beginning and ending with flour mixture, beating just until combined after each addition. Divide batter between prepared pans.

Bake for 30 minutes. Cover loosely with foil, and continue baking until a wooden pick inserted in center comes out clean, 30 to 45 minutes more. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks. Place parchment paper under racks, and pour Lemon Glaze over loaves. Garnish with thyme and lemon slices, if desired.