Slice peaches/nectarines and place in a 13 x 9 baking pan. Distribute them as evenly as possible. Sprinkle with lemon juice.
In your mixing bowl, cream butter and sugar together until sandy. Add flour, baking powder, and salt and mix well. With your mixer on low, slowly stream in milk. Turn the mixer up to medium, and beat for about two minutes, until light and fluffy.
Drop large spoonfuls of batter on top of the peaches, then spread it out in a relatively even layer (as much as possible) keeping the batter thickness to no more than 1/2". Sprinkle 1/2 cup of sugar evenly over the top, then drizzle hot water over the sugar.
Bake for 70-80 minutes, until the crust is a crispy, golden brown, and a toothpick inserted in the cracks comes out clean (check several different cracks, not just the middle, to ensure it's fully cooked). Let it sit for about 30 minutes before serving to let the juices thicken.