Preheat oven to 375°F. Spray bottom and sides of a 13 x 9-inch baking pan with cooking spray.
Combine oats, 1-1/2 cups flour, brown sugar, 1 teaspoon cinnamon and baking soda in a large bowl or in the bowl of your food processor. Stir or pulse to combine. Cut in 1 cup butter with a pastry blender or two knives until crumbs are the size of small peas. For food processor, pulse cold butter.
Divide streusel mixture into two equal halves. In one half, stir in 1 cup mozzarella cheese. Press cheese half (3 cups) of oat mixture firmly into baking dish. Bake in a preheated 375°F oven 10 minutes. Cool 5 minutes.
Combine blackberries, preserves, vanilla, remaining flour and cinnamon in a medium bowl. Stir in remaining 1/2 cup cheese, pecans (if using) and lemon peel; mix well. Spread mixture over crust. Sprinkle remaining oat mixture over blackberry mixture.
Bake 25 minutes or until topping is golden brown. Cool completely. Cut into bars.
Make the Glaze:
Mix one tablespoon of the milk into the powdered sugar. It may be a little stiff, but keep stirring until all dry sugar is gone. If need be, add milk one drop at a time to loosen the mixture. Add additional drops of milk until you reach a thick, but drizzling consistency. Drizzle over individual bars and allow to set up.
Notes
Don't try to cut or move these until they are completely cooled, otherwise they'll fall apart. They need to be cooled off in order to bind together.