Beat eggs, milk, white sugar, vanilla extract, and lemon juice in a bowl. Combine bread cubes, gingersnap crumbs, strawberries, rhubarb, and cashews in a separate bowl. Pour egg mixture over the bread cube mixture, stir to moisten, and let stand 10 minutes.
Grease a 9x13-inch baking dish and pour the bread mixture into the dish. Press down on the mixture to slightly compact the bread cubes and make a smooth top; cover dish and refrigerate 1 hour.
Preheat oven to 375 degrees F (190 degrees C).
Stir flour, rolled oats, brown sugar, and melted butter in a bowl until mixture is crumbly; spread oats mixture over the bread pudding. If necessary, lightly press topping onto the bread mixture. Cover dish with aluminum foil.
Bake in the preheated oven until the bread pudding is set, 40 to 45 minutes. Uncover, return to oven, and bake until pudding puffs up and begins to brown on top, 15 to 20 more minutes. Let cool for 30 minutes before serving.