Preheat your oven to 350°F (175°C).
In a large bowl, combine the mixed berries, sugar, and lemon juice. Let it sit for 10 minutes.
In another bowl, whisk together flour, baking powder, and salt.
Add the milk, melted butter, and vanilla extract to the dry ingredients and mix until just combined.
Pour the berry mixture into a greased baking dish.
Pour the batter over the fruit evenly.
Bake in the preheated oven for 40-45 minutes until the top is golden brown.
Check for doneness with a toothpick; it should come out clean.
Allow to cool slightly before serving.
Top with whipped cream if desired.