Preheat the oven to 400°F (200°C). Line a 12-cup muffin tin with liners or grease the cups.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk buttermilk, oil (or melted butter), egg, and vanilla until combined.
Pour the wet into the dry and stir gently until just combined. Do not overmix.
Toss the chopped strawberries and rhubarb with a light dusting of flour to prevent sinking, then fold into the batter.
Divide batter among muffin cups, filling 3/4 full. Sprinkle coarse sugar on top if desired. Bake at 400°F for 5 minutes, then reduce heat to 375°F (190°C) and bake 13–17 more minutes until golden and a toothpick comes out with moist crumbs.
Cool in the tin 5 minutes, then transfer to a wire rack. Store at room temperature in an airtight container for up to 3 days or freeze for longer storage.
Notes
If rhubarb is very tart, toss with 1–2 tbsp sugar and let sit 10 minutes before draining and folding in.
For dairy-free muffins, use almond or oat milk with 1 tbsp lemon juice and coconut oil or dairy-free butter.
To prevent fruit from sinking, toss it in flour before folding into the batter.