Preheat the oven to 375°F and line a 12-cup muffin pan with paper liners or lightly grease each cup.

In a small bowl, toss the strawberries and rhubarb with 1 tablespoon of sugar and let sit for 10 minutes to draw out excess juice.

Drain the fruit well, discarding the syrup, then gently toss the fruit with 1 tablespoon of flour to help suspend it in the batter.

In a large mixing bowl, cream together the softened butter and ½ cup sugar until light and fluffy.

Add the eggs one at a time, mixing well after each, then pour in the buttermilk and stir until smooth.

In a separate bowl, whisk together the 2 cups flour, baking powder, and salt.

Add the dry ingredients to the wet ingredients and mix until just combined.

Gently fold the floured strawberries and rhubarb into the batter, being careful not to overmix.

Divide the batter evenly among the muffin cups, filling each about ¾ full.

In a small bowl, stir together the flour, brown sugar, and salt for the streusel topping, then mix in the melted butter until the mixture forms coarse crumbs.

Sprinkle the streusel evenly over each muffin, pressing lightly to help it adhere.

Bake for 22–25 minutes, or until the muffins are golden and a toothpick inserted in the center comes out clean.