Cream together butter and sugar in the bowl of a stand mixer fitted with a paddle attachment. Add the eggs one at a time, beating well after each egg. Add in vanilla.
In a separate bowl combine the flour, salt, and baking powder. Gradually add the dry ingredients to the wet and mix until a soft dough forms. Press into a 18" x 13" x 1" baking pan into an even layer, adding extra up the sides to form a crust.
For the Filling:
Place the drained peaches* in a large bowl and add 1 cup sugar, 1/4 cup flour and 1/2 tsp cinnamon. Gently stir to coat and then pour over the pie crust.
For the Topping:
In a separate bowl combine 1 cup oats, 1 cup packed brown sugar and 1/2 cup of flour. Use a pastry cutter or two forks to work in the small pieces of butter until the mixture resembles coarse crumbs. Stir in the 1/2 cup sliced almonds. Sprinkle the mixture over the peaches and bake for 40 to 45 minutes, or until the filling is bubbly and the topping is golden brown.
Cool slightly in pan on a wire rack. Serve warm or cool completely.
*if using fresh peaches: (4 lbs. or 12 medium peaches) remove peel and slice thin before tossing in the sugar mixture. If using frozen peaches toss with the sugar mixture as directed, then let stand at room temperature for 30 minutes before adding to the crust.