Line 18 muffin tins with paper liners. Preheat oven to 350°F (180°C).
In a medium bowl, whisk together gluten-free flour, poppy seeds, baking powder, xanthan gum (if needed), and salt. Set aside.
In a large bowl, combine sugar, sour cream, melted butter, eggs, and lemon juice. Mix well with a handheld mixer or spatula until fully combined.
Fill each paper-lined muffin tin about ¾ full with the thick batter. Use a lightly greased ice cream scoop for even portions. Add coarse sugar on top if desired.
Bake for 22-24 minutes, or until a toothpick inserted into the center comes out clean. Let muffins cool completely in the tins before removing.
Glaze: In a small bowl, stir together powdered sugar and lemon juice until smooth. Drizzle generously over the cooled muffins.