In a large bowl, whisk together 2 cups all-purpose flour, 1½ teaspoons baking powder, 1 teaspoon salt, and ½ teaspoon baking soda.
In another large bowl, whisk together 1 egg, ¼ cup vegetable oil, 2 teaspoons orange zest, ¾ cup orange juice, and ¾ cup sugar. Gently stir in the flour mixture. Fold in 1¼ cup chopped rhubarb and ½ cup chopped pecans.
For the streusel:
In a medium bowl, whisk together ¼ cup sugar and 3 tablespoons flour. Slice 3 tablespoons cold butter into small cubes and work it into the sugar/flour mixture with your fingertips until the butter is the size of peas. Fold in ¼ cup chopped pecans.
Spray a nonstick 12-cup muffin pan with nonstick cooking spray. Divide the batter evenly among the muffin wells, until they are ¾ full. Sprinkle the batter with the streusel topping.
Bake for 20–25 minutes (I baked them for 23 minutes), or until lightly brown on top. Let sit in the pan for 5 minutes before removing the muffins from the pan.
Notes
Make sure the butter for the streusel is very cold (straight out of the refrigerator)—it creates the crunchiest streusel!