In a small saucepan, gently heat 1 cup of whole milk and 1/4 cup of unsalted butter until warm but not boiling, about 110°F. Stir in 2 1/4 teaspoons of instant yeast and 1/4 cup of granulated sugar, letting it sit for about 5 minutes until bubbly.

In a mixing bowl, combine the egg, milk mixture, 3 1/2 cups of all-purpose flour, and 1 teaspoon of salt. Mix until the dough is smooth and elastic, approximately 5 minutes.

Transfer the dough to a lightly greased bowl, cover it with a clean kitchen towel or plastic wrap, and place it in a warm area. Allow to rise until doubled in size, about 1 to 1.5 hours.

While the dough rises, prepare the rhubarb filling by cooking 2 cups of sliced rhubarb in a saucepan with 1/2 cup of granulated sugar, 1 tablespoon of lemon juice, and 2 tablespoons of cornstarch over medium heat. Stir continuously for about 10 minutes until thick and jam-like. Remove from heat and let cool.

Once the dough has risen, punch it down gently to release air. Roll out the dough into an 18 by 12 inch rectangle. Spread cooled rhubarb filling evenly over the dough, then roll it tightly from the long side. Cut into 12 equal rolls and place them in a greased 9x13 inch baking pan.

Cover the pan with a kitchen towel and let the rolls rise again in a warm place for about 30-45 minutes.

Preheat your oven to 350°F. Brush tops with the egg wash and bake for 25-30 minutes, or until golden brown.

Prepare the glaze by whisking together 1/2 cup of cooled rhubarb filling, 1 cup of powdered sugar, and 1 tablespoon of lemon juice until smooth.

Once baked, let rolls cool for about 10 minutes, then drizzle or pour the glaze over warm rolls.