Preheat oven to 400 degrees F.
In a small bowl, rub the lemon zest into the sugar using your fingers or a spatula until fragrant.
In a large bowl, sift together dry ingredients; flour, baking powder, salt, sugar, lemon zest, and poppy seeds. Using a pastry blender, cut in the butter to coat pieces with flour. Mixture should look like coarse crumbs. Make well in center and pour in heavy cream and lemon juice. Fold everything together; do not overwork.
When you have what looks like shreds of dough (almost like when you are working with pie crust), pour the dough out onto a floured surface and finish kneading it there until the dough comes together and forms a smooth ball. Pat the dough out into a rectangle and using a bench knife, cut the rectangle in half so that you have two squares. Cut each square in half, then cut down the middle of the rectangle so that you now have eight squares. Cut each square on the diagonal to get 16 triangles.
Using the bench knife, transfer each wedge to a baking sheet lined with parchment paper, making sure there is an inch or two separating each wedge. Brush the tops of the scones with heavy cream. Sprinkle with coarse sugar if desired.
Bake the scones for 12-17 minutes or until golden brown. Cool on the baking sheets for 2 minutes before transferring to a cooling rack. Allow scones to cool completely before brushing with glaze.
To make the glaze, stir together the powdered sugar and lemon juice in a small bowl. Add water or milk 1 tsp at a time and continue stirring until desired consistency. Brush the glaze onto the cooled scones.
Serving Size 1