Preheat your oven to 350°F and grease a 9x13 inch baking dish (or similar size cake pan) with butter or cooking spray. While the oven preheats, measure out all your ingredients and have them ready: this is especially important for this recipe since you'll be working quickly once you start mixing. Finely grate the lemon zest and set it aside—I like to do this early so the citrus oils stay fresh.

In a large mixing bowl, combine the softened butter, sugar, and lemon zest. Beat on medium-high speed for about 3 minutes until the mixture is pale and fluffy—this creaming process incorporates air into the batter, which helps the cake rise and creates a tender crumb. Stop and scrape down the sides of the bowl as needed to ensure everything is evenly combined.

Add the egg and vanilla to the creamed butter mixture and stir until fully incorporated, about 1 minute. Make sure the egg is completely mixed in—this helps emulsify the batter and ensures even texture throughout the cake.

In a separate bowl, whisk together the flour, baking soda, and salt. Now add these dry ingredients to the wet mixture in three additions, alternating with the yogurt: start with half the flour mixture, then add the full cup of yogurt, then finish with the remaining flour mixture. Stir gently after each addition until just combined—I find this gentle folding approach prevents overmixing and keeps the cake tender. The yogurt adds moisture and tang that complements the tart fruit beautifully.

Gently fold the rhubarb and strawberries into the batter until evenly distributed—be careful not to overmix or break up the fruit too much. While you're preparing the fruit, mix together the melted butter, cinnamon, and brown sugar in a small bowl to make the crumb topping. Pour the batter into your prepared pan, then sprinkle the cinnamon-sugar mixture evenly over the top. The melted butter will soak into the topping as it bakes, creating a delicious streusel-like crust.

Bake for 50–60 minutes, until the top is golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs (not wet batter). The cake should smell fragrant and the fruit juices may bubble slightly around the edges. Remove from the oven and let cool in the pan for at least 15 minutes before serving—this allows the cake to set and makes it easier to slice cleanly.