In a large bowl, combine eggs, milk, vanilla extract, and lemon zest. Mix thoroughly (we used our immersion blender). Slice challah into 1 inch cubes and gently toss with half of the blackberries and blueberries.
Lay down half of challah with fruit into a greased baking dish. Add half of the preserves and mascarpone, using a spoon to dollop them randomly. Pour over half of the egg mixture. Add remaining challah/berries, then the rest of the egg mixture, then the mascarpone/preserves, and finally sprinkle with sliced almonds.
Place in refrigerator and chill for 1 hour. When ready, preheat oven to 350 degrees. Bake strata for 1hr and 15 minutes or until golden brown. Serve warm or cold.