Preheat your oven to 400°F (200°C). Cut cold butter into small cubes, quarter strawberries, and cut rhubarb into ½-inch pieces.

In a food processor, combine all-purpose flour, baking powder, and salt. Pulse until mixed. Add cubed butter, pulsing until resembling coarse breadcrumbs.

Transfer mixture to a bowl. Stir in granulated sugar, ground cinnamon, and lemon zest. Gently fold in rhubarb and strawberries.

Pour in milk and mix until a shaggy dough forms. Be careful not to overmix; adjust flour as needed.

Turn dough onto floured surface, shape into a 10-inch circle, about 1-inch thick. Cut into 8 wedges.

Place wedges on parchment-lined baking sheet. Whisk egg and water for egg wash; brush tops.

Bake for 18-20 minutes or until golden brown. Check doneness with a toothpick.

Let cool on a wire rack. Mix icing sugar, milk, and vanilla extract for glaze; drizzle over warm scones.