In a large bowl, combine flours, brown sugar, baking powder, baking soda, cinnamon, salt, and lemon zest
In another bowl, whisk together eggs, milk & lemon mixture, oil, and vanilla
Make a well in the center of the dry ingredients and pour in the egg/milk mixture, stirring with just a few swift strokes to just moisten the ingredients. Don't over mix.
Fold in the rhubarb and strawberries
Spoon into muffin pans, should fill 18 holes.
In a small bowl, put the topping together by combining the nuts, cinnamon, & brown sugar
Cut in butter until crumbly
Sprinkle topping over each muffin evenly
Bake for 25-30 minutes or until tops are golden and a toothpick inserted in the center comes out clean.
Notes
If you don't have buttermilk, substitute an equal quantity of milk, with 2 tbsp lemon juice or vinegar stirred in. Allow to sit for 5 minutes before using.