Preheat your oven to 350°F (175°C). This ensures that the bread bakes evenly and rises properly.

Grease a 9x5 inch loaf pan with butter or line it with parchment paper for easy removal.

In a large mixing bowl, whisk together the all-purpose flour, whole wheat flour, granulated sugar, baking powder, baking soda, and salt. This helps to evenly distribute the leavening agents and ensures a uniform texture.

In another bowl, cream the softened butter until light and fluffy using a whisk or electric mixer. This incorporates air into the butter, which helps the bread rise.

Add the eggs one at a time, mixing well after each addition.

Stir in the buttermilk, lemon zest, and lemon juice until just combined. Avoid overmixing to keep the bread tender.

Gradually add the wet ingredients to the dry ingredients, folding gently with a rubber spatula until just combined. Do not overmix; a few lumps are okay.

Gently fold in the diced rhubarb, ensuring it's evenly distributed throughout the batter.

Pour the batter into the prepared loaf pan and smooth the top with a spatula.

Bake in the preheated oven for 55-65 minutes, or until a toothpick inserted into the center comes out clean. This ensures the bread is fully cooked inside.

Once baked, remove from the oven and let it cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

In a small bowl, whisk together the powdered sugar and lemon juice until smooth. This glaze adds a sweet and tangy finish to the bread.

Once the bread is completely cool, drizzle the glaze over the top. Allow it to set for a few minutes before slicing.