Preheat oven to 375°F. Coat an 11-by-7-inch glass baking dish or a 2-quart casserole with cooking spray.
Whisk egg whites, eggs and milk in a medium bowl. Add sugar, vanilla and orange zest: whisk to combine.
Toss bread, gingersnaps, strawberries, rhubarb and 1/4 cup walnuts in a large bowl. Add the custard and toss well to coat. Transfer to the prepared baking dish and push down to compact. Cover with foil.
Bake until the custard has set, 40 to 45 minutes. Uncover, sprinkle with 1/4 cup walnuts (or Streusel Topping) and continue baking until the pudding is puffed and golden on top, 15 to 20 minutes more. Transfer to a wire rack and cool for 15 to 20 minutes before serving.
Spread walnuts on a baking sheet, place in a preheated 350 degree F oven and toast, stirring once, until fragrant and lightly browned, 7 to 9 minutes.To make streusel topping: Combine 1/3 cup flour, 1/4 cup oats (preferably old-fashioned), 2 tablespoons brown sugar and 2 tablespoons canola oil in a small bowl. Spread the mixture on top of the pudding after the initial 40 to 45 minutes of baking.