Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
In a large bowl, whisk together flour, granulated sugar, brown sugar, baking powder, baking soda, and salt.
In another bowl, mix buttermilk, egg, oil, and vanilla extract until smooth.
Pour wet ingredients into dry ingredients and stir gently until just combined (do not overmix).
Toss diced strawberries with 1 tbsp flour (to prevent sinking), then fold into the batter.
Pour batter into the loaf pan. Bake for 50–60 minutes, or until a toothpick inserted comes out clean.
Let cool in the pan for 10 minutes, then transfer to a wire rack. Slice once fully cooled.
