Start by preparing both components that will top your focaccia. First, combine the sliced rhubarb, strawberries, sugar, cornstarch, lemon juice, and vanilla extract in a saucepan over medium heat. Simmer for about 5 minutes, stirring occasionally, until the fruit begins to break down and release its juices—this concentrates the flavor and thickens the filling naturally. Pour the filling into a bowl and let it cool to room temperature while you prepare the crumble. In a separate bowl, combine the flour, sugar, and ground ginger for the topping. Cut the cold butter into small cubes and work it into the flour mixture using your fingertips or a pastry cutter until the mixture resembles coarse breadcrumbs—the cold butter creates those desirable crumbly pockets. I like to refrigerate the crumble mixture for at least 15 minutes while you work on the dough; this keeps the butter cold and ensures better texture.

In a large mixing bowl, combine the flour, sugar, salt, and yeast. Pour in the warm water (110°F to 115°F is crucial for proper yeast activation) and mix until a shaggy dough forms. Once combined, knead the dough for 5 to 8 minutes by hand or in a stand mixer until it becomes smooth and elastic. The dough should feel soft but not sticky—if it's too wet, dust with a little flour. After kneading, let the dough rest in a covered bowl for 20 minutes; this allows the gluten to relax and makes stretching easier later.

Preheat your oven to 375°F. While it heats, melt the 5 tablespoons of butter in a 9x13-inch baking dish (or use a large baking sheet if you prefer a thinner focaccia). Tilt the pan to coat the bottom evenly with butter. Remove the rested dough from Step 2 and place it in the buttered pan. Begin stretching it gently to fit the pan dimensions, working from the center outward and being patient with the dough—it will resist at first, so let it relax for a minute and stretch again if needed.

Spread one-third of the cooled fruit filling from Step 1 over one-third of the stretched dough (lengthwise). Sprinkle one-third of the crumble mixture from Step 1 over the fruit. Fold the opposite third of the dough over the top to cover the filling. Now spread another third of the fruit and crumble on the exposed center section, then fold the remaining end of dough over it, creating a layered fold pattern. Rotate the pan 90 degrees and gently stretch the dough to fit the pan again. Top with the remaining fruit filling and crumble topping, distributing it evenly across the surface. I find this folding technique creates beautiful ribbon-like layers when the focaccia is cut and sliced, plus it keeps the filling secure during baking.

Cover the pan loosely with a damp towel and let the focaccia rise in a warm spot for 30 minutes—this second rise gives the bread a lighter, airier crumb. After 30 minutes, transfer the pan to your preheated 375°F oven and bake for 40 to 45 minutes if using a 9x13-inch pan, or 30 to 35 minutes on a baking sheet, until the top is golden brown and a toothpick inserted into the dough (avoiding the fruit) comes out clean. The edges should pull away slightly from the pan sides.

While the focaccia bakes, whisk together the confectioners sugar and freshly squeezed lemon juice in a small bowl until smooth—aim for a pourable but not too-thin consistency. As soon as the focaccia comes out of the oven, drizzle the glaze evenly over the warm bread. The warmth of the bread helps the glaze set lightly and creates a beautiful finish. Let it cool for 5 to 10 minutes before slicing and serving.