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Preheat oven to 400 degrees F.
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Spray a cake pan or small casserole dish with cooking spray.
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Combine rhubarb, strawberries, sugar, lemon zest, blueberry balsamic vinegar and black pepper into a sauce pan and cook over medium heat, while stirring occasionally, until the rhubarb starts to breakdown, about 10 minutes.
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Set rhubarb strawberry mixture aside and let cool.
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In a mixing bowl, combine whole wheat flour, oats, sugars and salt. With your hands, work in the butter until large clumps form.
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Spoon rhubarb strawberry mixture into bottom of greased cake pan or casserole dish and evenly spread crumble topping over rhubarb strawberry mixture. Bake in preheated oven for 30 - 35 minutes, until golden brown.