Start with the blueberry filling first since it needs time to cool before assembly. Combine 12 oz fresh blueberries, 1/4 cup sugar, and 1 tbsp lemon juice in a saucepan over medium heat. Bring to a simmer and cook for 5-10 minutes until the mixture thickens and the berries break down slightly. In a small bowl, mix 1 tbsp cornstarch with 2 tbsp water to create a slurry, then stir it into the simmering blueberries. Cook for another 1-2 minutes until the mixture reaches a jam-like consistency, then remove from heat and let cool completely while you work on the dough. I always save the blueberry filling in a separate area so I don't accidentally use it all during assembly.

Gently warm 1 1/4 cups milk with 9 tbsp butter in a saucepan over low heat until the mixture reaches 100-110°F (this temperature is ideal for yeast activation—too hot and you'll kill the yeast, too cool and it won't activate properly). Remove from heat and stir in 1/4 cup brown sugar, 1 3/4 tsp salt, 1 tbsp vanilla, and 1/4 oz (about 2 1/4 tsp) yeast, mixing until the yeast is fully dissolved. Add 2/3 cup of active, bubbly sourdough starter to the wet mixture and combine well. In a stand mixer bowl, add 4 cups flour and pour in the wet ingredients. Mix on low speed until a shaggy dough forms, then increase to medium speed and knead for 8-10 minutes until the dough becomes smooth and elastic.

Transfer the dough from Step 2 to a lightly oiled bowl, cover with a damp towel or plastic wrap, and let it rise at room temperature for 1 hour until it roughly doubles in size. While the dough is rising, prepare your filling components: melt 4 tbsp butter and let it cool to room temperature, then combine 1/2 cup brown sugar, 1 1/4 tsp freshly ground cinnamon, and 1/4 tsp ground nutmeg in a small bowl. By the time your dough has finished rising, all your filling components will be ready to use.

Turn the risen dough from Step 2 onto a floured surface and roll it into a 13x21 inch rectangle (if the dough resists, let it rest for 5 minutes before continuing). Brush the entire surface with the cooled melted butter from Step 3, then evenly sprinkle the cinnamon-sugar mixture from Step 3 over the butter. Reserve 1 1/2 tbsp of the cooled blueberry filling from Step 1 for the frosting, then spread the remaining blueberry filling in an even layer over the cinnamon sugar. I like to leave a small 1/2-inch border along one of the long edges to make rolling easier.

Starting from the long edge opposite your border, tightly roll the dough into a cylinder, then cut it into 12 equal pieces (a bench scraper or serrated knife works best for clean cuts). Arrange the rolls cut-side up in a 9x13 inch baking dish, spacing them evenly so they have room to rise without touching. Cover the pan loosely with plastic wrap or a damp towel and let the rolls rise at room temperature for 1 hour until they're puffy and nearly touching each other.

Preheat your oven to 350°F while the rolls finish their second rise. Once the rolls are puffy, place the baking dish in the preheated oven and bake for 30-35 minutes until the tops are golden brown and the internal temperature reaches 190°F when measured with a thermometer in the center of a roll. Remove from the oven and let cool for 5 minutes while you prepare the frosting.

While the rolls are still warm, beat 4 tbsp butter and 4 oz cream cheese (Philadelphia brand works beautifully here) together in a mixer until light and fluffy, about 2-3 minutes. Add 1 1/2 cups powdered sugar, 1 tsp vanilla, and the reserved 1 1/2 tbsp blueberry filling from Step 4, then beat until smooth and spreadable. Generously spread the frosting over the warm rolls, allowing it to melt slightly into the crevices. Serve immediately for the best flavor and texture.