Preheat oven to 350°F (175°C) and grease a 12-cup muffin pan.

In a medium bowl, combine flour, oats, brown sugar, cinnamon, and turbinado sugar for the crumble.

Cut in cold unsalted butter until mixture resembles coarse crumbs. Set aside.

In a large mixing bowl, whisk together all-purpose flour, brown sugar, oats, baking soda, salt, and cinnamon.

In a separate bowl, mix together milk, vinegar, vanilla extract, vegetable oil, and egg until well combined.

Pour wet ingredients into dry mixture, folding gently until just combined.

Gently fold in sliced rhubarb.

Scoop batter into greased muffin cups, filling each about 2/3 full. Top generously with crumble and add a piece of rhubarb on top.

Bake for 20-25 minutes, or until muffins are golden brown and a toothpick inserted comes out clean.

Let muffins cool in the pan for about 10 minutes before transferring to wire rack.