Make the Dough:

1. In a small saucepan set over medium heat, combine milk and butter. Heat until the butter is mostly melted, then remove from heat and continue stirring until the butter is completely melted. Let cool for 5 minutes, or until lukewarm.
2. Pour into the bowl of a stand mixer that's been fitted with a dough hook. Add the eggs, one at a time, then sprinkle in the sugar, yeast and salt. Turn the mixer on at low speed, then add flour and knead for 2-3 minutes or until the dough starts to come together. Increase the speed to medium, and continue kneading for about 10 minutes or until the dough is soft and smooth and completely pulls away from the sides of the bowl.
3. Transfer the dough to a large lightly-oiled mixing bowl and cover tightly with plastic wrap. Place in a warm, draft-free spot to rise until doubled, about 1 hour. (Alternatively, you can leave the dough to rise in the refrigerator overnight if preparing the night before.)
4. Punch down the dough, then divide into 16 equal size portions and shape into balls. Arrange on two large parchment-lined baking sheets, and cover loosely with plastic wrap. Let rise until doubled again, about 1 to 1 1/2 hours.

Prepare the Filling:

1. As soon as you've put away the buns to rise, prepare the filling. In a small mixing bowl, combine the strawberries, rhubarb, brown sugar and cinnamon. Cover and set aside to macerate while the buns finish rising.

Prepare the Streusel:

1. Next, prepare the streusel by stirring together the flour, brown sugar and cinnamon in a small bowl. Using your fingers, rub in the butter until the mixture is coarse and crumbly. Cover with plastic wrap and chill until ready to use.

Fill and Bake the Buns:

1. Preheat oven to 350F. Using the bottom of a glass or measuring cup with a 2" base, press down into the top of each ball to make a deep indent. Brush the buns all over with egg wash.
2. Using a small spoon, scoop about 2 tbsp of filling into each of the buns, trying to leave most of the liquid behind in the bowl, then sprinkle generously with streusel.
3. Bake in preheated oven for 20 to 25 minutes, or until pale golden brown. (Do not bake any longer... you want these to be slightly underbaked.) Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool off for at least 20 minutes. Serve immediately - these are best eaten the day they're made, if at all possible, though they will keep for another day or two in a tightly covered container.