Preheat your oven to 350°F (175°C). Grease and flour a standard 9x5-inch loaf pan, or line it with parchment paper.

In a large bowl, whisk together the flour, 1 cup of sugar, baking soda, salt, and cinnamon until well combined.

In a separate medium bowl, whisk together the beaten egg, milk, melted butter, and vanilla extract.

Pour the wet ingredients into the dry ingredients. Mix gently with a spatula until just combined. Do not overmix; a few streaks of flour are okay. Gently fold in the chopped rhubarb.

Pour the batter into the prepared loaf pan. Sprinkle the remaining 1 tablespoon of granulated sugar evenly over the top of the batter.

Bake for 50 to 60 minutes, or until a wooden skewer inserted into the center comes out clean. If the top begins to brown too quickly, loosely tent it with aluminum foil after 40 minutes.

Let the bread cool in the pan for 10 minutes before carefully turning it out onto a wire rack to cool completely before slicing.