Cream butter, sugar and lemon zest together until light and creamy, about 3 minutes. (by hand with a wooden spoon or using an electric mixer)
Add the eggs one at a time, fully incorporating. Make sure to scrape down the sides of the bowl
Mix together the flour, baking powder, baking soda and salt in medium-sized bowl.
Add half the flour mixture to the creamed, mixing just enough to incorporate
Add Greek yogurt, mixing in just enough to incorporate
Add the remaining flour mixture, mixing just enough so it’s barely incorporated (there will still be dry bits of unincorporated flour)
Using a flexible spatula, fold the blueberries into the mixture. Being careful not to overmix.
Scrape the batter into the prepared baking pan and smooth the top.
Add the topping over the batter and bake for 50-55 minutes at 350 degrees. Or until the buckle is lightly browned on top and feels just set in the center. It should spring back lightly when you touch the center.
When the buckle is almost finished baking, make the syrup by heating the lemon juice and sugar in a small saucepan over medium heat, cooking it until it thickens. It should take about 5 minutes. Don't allow the syrup to brown.
Remove the buckle from the oven and pour the warm lemon syrup over it, letting it soak in.
Serve buckle slightly warm or at room temperature with vanilla ice cream or home made whipped cream.