For making beetroot puree, pressure cook one large or two medium beetroots for 3 whistles till done. Allow to cool, peel off the skin, chop into large pieces and puree in your blender till fine. Add 3-4 tbsp water if necessary. Use 3/4 cup from this puree. I got almost 1 1/2 cup of puree from blending one fairly big beetroot.
Instead of milk+vinegar, you can use whisked yogurt.