Preheat your oven to 350°F.
In a large mixing bowl, combine the strawberries, rhubarb, 3/4 cup of sugar, cornstarch, vanilla extract, and lemon juice. Toss until the fruit is well coated.
Pour the fruit mixture into a greased 9x13-inch baking dish.
In another bowl, whisk together the flour, baking powder, salt, and remaining 1/4 cup of sugar.
In a separate bowl, mix the melted butter, milk, and egg until well combined.
Add the wet ingredients to the dry ingredients and stir until just combined. Do not overmix.
Spoon the batter over the fruit mixture in the baking dish, spreading it out as evenly as possible.
Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and a toothpick inserted into the batter comes out clean.
Let the cobbler cool for about 10 minutes before serving.