Butter the bottom and sides of a 2-2 1/2 quart baking dish with sides over 2 inches high. Preheat the oven to 375 degrees F.
Peel the peaches. You can do this with a paring knife or vegetable peeler one by one, or you can bring a medium size pot of water to boil (with water deep enough for peaches to totally submerge later on). When the water is boiling, drop the peaches in for 10 seconds, then move them to a bowl of iced water. The skins will easily slip off.
Squeeze the lemon juice into the bottom of a large bowl. Cut each peach into 1/2-inch slices, dropping the slices directly into the lemon juice. After every 2 or 3 peaches, toss the slices in the juice to help prevent discoloring. Discard the stones.
Mix the flour and sugar together, and then sprinkle half over the peach slices. Gently stir the flour mixture into the peaches as you add the second half of the flour mixture. It should coat all the slices. Transfer the peaches to the baking dish.
Sprinkle the topping mixture over the peaches, trying to distribute it evenly and cover the top of the fruit. Place the baking dish on a baking sheet to catch any juices that might overflow.
Bake the crisp for 45 minutes, until the topping is golden brown and the juices from the fruit are bubbling around the edges.
Cool on a wire rack for about an hour before serving.