Combine the milk, zest and lemon juice in a tall glass. Refrigerate and allow to curdle for 10 to 15 minutes, until thick. Sift together the flour, a generous pinch of salt, and baking soda into a medium bowl. Using your fingers gently mix in the butter until an even, but grainy texture is reached, being careful not to melt the butter as you work. Add the soured milk and scallions and stir until the dough comes together. (Dough will be sticky.) Gather the dough into a ball, and refrigerate 15 to 20 minutes, until chilled.
Transfer the dough to a floured surface and knead 3 to 4 times until the dough is smooth. For fluffy biscuits do not over knead the dough! Roll out the dough to about 3/4-inch thickness. Cut with a 3-inch pastry ring or the top of a glass. Arrange the biscuits on a parchment-lined sheet pan. Allow the biscuits to slightly touch and gently adhere when baked. Bake until biscuits are golden, about 15 minutes. Remove from the oven and transfer to a bread basket or bowl and serve.