Mix watercress, onions and garlic in a bowl and set aside. Preheat oven to 425F. Grease a baking sheet. In a large bowl, combine dry ingredients and whisk together thoroughly. Cut in butter until the mixture is crumbly. Add the watercress mixture and toss with fork to combine. Make a well in the center of mixture and pour in milk.Toss quickly with fork until dough holds together, then turn out onto floured surface and knead lightly about 15 times. Roll out to 1/2-inch thickness and cut with a 2-inch floured biscuit cutter. (Use a glass or a clean, empty can if you don't have a cutter or cut into squares.) Transfer biscuits to the prepared baking sheet and bake 12-15 minutes until golden brown and done in the center.