Preheat your oven to 375°F (190°C). If using muffin liners, line a 12-cup muffin tin.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
In another bowl, mix the melted butter, egg, vanilla extract, and buttermilk until smooth.
Slowly pour the wet ingredients into the dry ingredients, and gently fold together using a rubber spatula until just combined. Do not overmix.
Fold in the chopped strawberries and rhubarb until evenly distributed throughout the batter.
Divide the batter evenly among the muffin cups, filling each cup about ⅔ full.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for about 5 minutes, then transfer them to a wire rack to cool completely.