Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease well with cooking spray.
Combine all dry ingredients in a large bowl – whisk together the flour, sugar, baking powder, baking soda, and salt until well mixed.
Mix the wet ingredients in a separate medium bowl – thoroughly whisk the Greek yogurt, eggs, oil, and vanilla extract until smooth and no yogurt lumps remain.
Pour the wet mixture into the dry ingredients and fold gently with a rubber spatula just until combined. The batter should be thick and not completely smooth – overmixing will create tough muffins.
Gently fold in the diced rhubarb and lemon zest with just a few strokes to distribute evenly throughout the batter.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full for that perfect dome.
Sprinkle the tops with coarse sugar for a delightful crunch and extra sweetness that balances the tart rhubarb.
Bake for 18-22 minutes until the tops are golden and a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely – though they’re absolutely divine while still warm!