Preheat the oven to 350°F (175°C) and grease a standard loaf pan with nonstick spray.

In a large bowl, whisk melted butter, sugar, and orange zest. Add Greek yogurt and mix well. Incorporate eggs one at a time, whisking after each.

In another bowl, sift together flour, baking powder, baking soda, and salt. Whisk until combined.

Fold dry ingredients into wet ingredients until just combined.

Gently fold in diced rhubarb.

Pour batter into the prepared loaf pan and sprinkle coarse sugar on top if desired.

Bake for about 1 hour, checking doneness with a toothpick; it should have a few moist crumbs.

Cool for 10 minutes in the pan, then transfer to a rack to cool completely before slicing.