Preheat oven to 375°F. Line 12 muffin cups with silpat liners or grease with butter.
To make the streusel, stir together the all-purpose and brown flour, sugars, cinnamon and salt. Stir in butter until mixed thoroughly and set aside.
In a large bowl, whisk the egg with the brown and white sugars. Whisk in the butter, then add the yogurt and mix. In a separate bowl, mix together the flours, baking powder and baking soda. Stir them gently into the yogurt mixture, mixing until just combined and still a bit lumpy. Fold in rhubarb/strawberry mixture and about 1/3 of the streusel mixture. I found the resulting mixture to be very thick.
Divide the batter evenly between the prepared muffin cups. Sprinkle each muffin with remaining streusel, then gently press the crumbs into the batter so that they bake into the muffin.
Bake for 15 to 20 minutes or until tops are lightly browned and a toothpick inserted into the center of muffins comes out clean. Place on a cooling rack for a few minutes, then remove muffins to finish cooling.
Adapted from Smitten Kitchen
Recipe by Just a Smidgen at https://justasmidgen.com/2012/04/13/oh-streusel-crumbs/