Combine yeast and brown sugar in a bowl and then slowly pour warm water in the bowl while stirring everything together at the same time. Allow 10 minutes for the mixture to sit and activate.
While waiting, melt the butter and milk together over medium heat, stirring pretty frequently until they are melted, being careful that the mixture doesn't get too hot. If it does by accident, allow it to sit for a few minutes before doing anything with it.
Whisk egg in a separate bowl.
Once the 10 minutes have passed, add the melted butter, milk and egg to the bowl and stir to combine.
In a large bowl, combine flour and salt; stir, and then add wet ingredients to dry ingredients, using your hands to knead the dough for approximately 3 minutes.
Line a separate large bowl with a bit of coconut oil or nonstick spray and transfer dough to the bowl. You can either cover it with plastic wrap and allow it to sit in a warm place for 1 hour OR, if your dough doesn't seem to be rising at room temperature, you can heat your oven to 200°, turn it off, and place the covered bowl (I use an oven-proof glass bowl plus a dish towel to cover it) into the oven for 1 hour. Ideally, you want the dough to double in size.
While you're waiting for the dough to rise, make the filling and start the icing, too.
Add the coconut oil, strawberries and rhubarb to a skillet and heat for 2-3 minutes, or until fruit is slightly soft. Pour into a bowl and set aside.
To begin the icing, add coconut oil, strawberries, rhubarb, sugar, and cinnamon to a small saucepan and heat over medium-high. Use a wooden spoon to break down the strawberries and rhubarb and, once mixture comes close to a boil, you can turn the heat to a simmer. Allow mixture to simmer for 10 minutes before removing from the heat and transferring to a blender. Wait a few minutes for mixture to cool before blending everything together; set aside.
Once dough has doubled in size, it's time to make your cinnamon rolls! Flour a baking mat and/or table completely and then roll dough out into an 8"x12" rectangle, approximately.
Brush melted butter atop dough, leaving a one inch border around the edges. Add sugar and cinnamon, using a spoon or your fingers to pat everything gently into the dough. Last, add chopped and cooked strawberries and rhubarb.
Gently roll the dough tightly long ways (the dough should look like a snake, not a log). Use a knife or dental floss to cut the dough into 1" cinnamon rolls, placing them in a greased cast iron skillet/cake pan/baking dish.
Once all the cinnamon rolls are cut, place plastic wrap or a towel overtop and allow them to sit at room temperature OR heat your oven to 200°, turn it off, and place the rolls in there for 30 minutes or until they've doubled or almost doubled in size.
Next, heat the oven to 350°. Once heated, brush cinnamon rolls with a bit of melted butter and place them uncovered in the oven and bake for 25-30 minutes.
While cinnamon rolls are baking, finish making the icing. Stir 1/4 of the blended rhubarb mixture with 1 cup of powdered sugar in a bowl with an electric mixer. Feel free to add more blended strawberry/rhubarb and/or more powdered sugar depending on how thick you want the icing to be.
Once rolls have finished baking, remove them from the oven, wait 5 minutes or so and then add the icing.