Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper. In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.

Add the very cold, cubed butter to the dry ingredients. Use a pastry blender, two forks, or your fingertips to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.

Gradually add 3/4 cup of very cold heavy cream to the flour mixture, mixing gently with a spoon or your hands until just combined. Do not overmix.

Gently fold in the diced strawberries and rhubarb until evenly distributed. The dough will be shaggy and a bit sticky.

Turn the dough out onto a lightly floured surface. Gently pat the dough into an 8-inch round, about 1 1/4 inches thick. Avoid overworking the dough to keep the scones tender.

Using a sharp knife or a bench scraper, cut the round into 8 wedges.

Carefully transfer the scone wedges to the prepared baking sheet, leaving some space between them. In a small bowl, whisk together the remaining 1 tablespoon of heavy cream and the egg (if using for egg wash). Lightly brush the tops of the scones with the egg wash.

Bake for 18-22 minutes, or until the tops are golden brown and a skewer inserted into the center comes out clean.

Remove from the oven and let cool on the baking sheet for 5-10 minutes, then transfer to a wire rack to cool completely before glazing.

While the scones cool, prepare the glaze. In a small bowl, whisk together the powdered sugar and 2 tablespoons of milk (or lemon juice) until smooth. Add more milk, 1 teaspoon at a time, if needed to reach a pourable but not too thin consistency.

Once the scones are completely cool, drizzle the glaze over the tops. Let the glaze set for a few minutes before serving. Enjoy warm or at room temperature.