Preheat the oven to 400 degrees (200C) with a rack positioned in the middle. Oil a 6-cup muffin tin and fill with muffin liners or tulip-shaped parchment liners.
Place the ingredients for the streusel in a bowl and mix until it forms large, moist crumbs. Set aside.
Put the strawberries, rhubarb, cornstarch, brown sugar and lemon juice in a bowl and gently stir to combine. Set aside while you mix the rest of the batter.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt and grated zest. Make a well in the center and stir in the egg, oil, butter, buttermilk and vanilla. Mix until the batter just comes together and the flour is incorporated.
Divide the batter evenly into the muffin cups. Spoon the strawberry mixture evenly over the batter (about 1/2 cup for each cup), then top with the streusel crumbs (2 tablespoons or so per cup)
Bake 22 - 27 minutes, until the muffins are golden brown on top and a toothpick inserted into the center of the muffins comes out with a few moist crumbs.
The recipe makes 6 jumbo "Texas" style muffins or 12 standard-size muffins.The muffins keep well for a few days, wrapped or stored in a covered container.