Preheat oven to 350 degrees F. Grease 12 standard muffin cups with butter or nonstick cooking spray or alternatively, line with paper liners.

Make the crumble topping: in a medium bowl combine rolled oats, brown sugar, flour, brown sugar and cold butter. Work it with your fingers until the mixture comes together to a crumbly texture. Set aside.

Prepare the filling: In a medium bowl add the rhubarb, strawberries, sugar, lemon juice and flour. Stir gently to combine and set aside.

Make the muffin batter: In a medium bowl whisk the eggs then add the milk, vanilla, melted butter and brown sugar. Whisk until combined and smooth.

In a large mixing bowl mix together the all purpose and whole wheat flours, rolled oats, baking powder, baking soda and salt.

Add the wet ingredients to the dry ingredients in the large mixing bowl. With a spatula stir to combine just until the flour is all absorbed.

Gently fold in the strawberry rhubarb mixture stirring just to combine.

Fill prepared muffin cups about ¾ full. Pile the crumble mixture on top of each muffin packing it with your fingers.

Bake for 25 - 30 minutes. Let cool for 5 minutes then gently un-mold the muffins and cool completely on a rack.