Prepare your ingredients and workspace. Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Keep butter in the refrigerator until the last possible moment to ensure it stays cold.
Combine the dry ingredients. In a large bowl, whisk together the flour, sugar, baking powder, and salt until well blended.
Cut in the butter using a pastry cutter or two knives until the mixture resembles coarse crumbs with some pea-sized pieces remaining. The butter must remain cold for flaky scones, so work quickly and with cold hands if possible.
Gently fold in the strawberries, rhubarb, and lemon zest with a rubber spatula, being careful not to crush the fruit. Distribute the fruit evenly throughout the flour mixture.
Whisk together the wet ingredients in a separate small bowl. Combine the heavy cream, egg, and vanilla extract until smooth.
Create a well in the center of your dry ingredients and pour in the liquid mixture. Stir with a fork just until the dough begins to come together. Do not overmix – the dough should look shaggy and slightly dry.
Turn the dough onto a lightly floured surface and gently pat it into a circle about 1-inch thick. Avoid kneading or overworking the dough, which will develop gluten and make tough scones.
Cut the dough into 8 equal triangles using a sharp knife or bench scraper, and transfer them to your prepared baking sheet, leaving about 2 inches between each scone.
Brush the tops with heavy cream and sprinkle generously with additional granulated sugar for a beautiful, crunchy crust.
Bake for 22-25 minutes until golden brown, rotating the pan halfway through for even baking. The scones should be firm to the touch and golden at the edges.
Cool on the baking sheet for 5 minutes before transferring to a wire rack. Serve warm or at room temperature.