Add the peaches, brown sugar, lemon zest, lemon juice and water to a shallow skillet and bring to a boil, stirring occasionally. Reduce heat to medium and continue to cook for about 3-5 minutes, until sugar is melted and peaches are saucy. Whisk together the cornstarch and water and pour the slurry into the peaches, stirring constantly. Cook for one minute until the peach mixture thickens. Remove from heat to cool.
ASSEMBLE THE CRUST & STREUSEL
In a large bowl, combine the oatmeal, flour, brown sugar, baking soda, salt and cinnamon. Stir to combine. Add the melted butter to the oatmeal mixture and stir until well combined.
Add 2 cups of the crust mixture to a 11 x 7" baking pan. Press the crust into an even layer in the bottom of the pan, leaving no gaps.
Add the sliced almonds to the remainder of the crust mixture and toss to combine.
ASSEMBLE THE BARS:
Spread the cooled peach mixture in an even layer over the crust. Sprinkle the streusel mixture loosely over the peach filling. Bake for 30 minutes or until golden brown. Let the bars cool to room temperature before slicing into squares. You should get about 24 1 1/4" bars.