for the filling
1 cup strawberries, stems removed and quartered
1 stalk rhubarb (approx 1 cup), peeled and diced
2 tablespoons butter
1/2 cup sugar
pinch of salt
1 tablespoon corn starch mixed with 2 tablespoons water
for the muffins
1 1/2 cups all-purpose flour
3/4 cup sugar
1/2 teaspoon salt
2 teaspoons baking powder
1/3 cup butter, melted and cooled slightly
1 egg, lightly beaten
2/3 cup milk
For the streusel topping
1/2 cup sugar
1/4 cup all-purpose flour
1/4 cup butter, cubed
2 teaspoons ground cinnamon
First make your filling. This can be made up to 2 days in advance and refrigerated. In a medium pan over medium heat, melt the butter and then add the strawberries, the diced rhubarb and sugar. Sauté until they are tender, about 3-5 minutes, before the corn starch slurry. Let come to a boil to thicken. Remove from the heat and set aside to cool.
Preheat oven to 400F. Grease muffin cups or line with muffin liners.
To make streusel topping: Mix together sugar, flour, butter, and cinnamon. Mix with fork until it looks like coarse sand.
Combine flour, sugar, salt and baking powder in one bowl. In another small bowl, combine melted butter, milk and egg. Mix this with flour mixture. Fill muffin cups about 1/3 of the way full and then create a small indentation. Fill with a tablespoon of the strawberry rhubarb mixture. Fill remainder of the way with the batter, right to the top, and sprinkle with streusel topping mixture.
Bake for 20 to 25 minutes in the preheated oven, or until a toothpick come out clean and they are a golden brown.