Preheat oven to 350°. Line 18 wells of a regular-size muffin pan with paper liners.
In a small bowl, stir together ¼ cup flour, oats, ¼ cup brown sugar, and cinnamon. Using a pastry blender or a fork, cut in grated butter until mixture is crumbly. Stir in pecans. Set aside.
In a large bowl, beat together vegetable oil and remaining 1½ cups brown sugar with a mixer at medium speed until well blended. Beat in eggs until smooth. Beat in yogurt, vanilla extract, baking powder, baking soda, and salt. Beat in remaining 2½ cups flour, ½ cup at a time, until batter is well blended and smooth. Stir in rhubarb.
Spoon batter into prepared muffin cups until three-fourths full. Sprinkle pecan mixture evenly over batter.
Bake until muffins are golden brown and a wooden pick inserted in centers comes out clean, 25 to 30 minutes. Let cool slightly before removing from muffin pan.
Recipe by TeaTime Magazine at https://teatimemagazine.com/a-new-case-is-brewing-in-the-latest-daisys-tea-garden-mystery/