First, mix your rhubarb and strawberries in a large bowl. Mix in the brown sugar and a 1/3 cup of granulated sugar, 1/2 tsp salt, lemon juice, and vanilla extract. Mix well.

Heat a medium-sized sauté pan over medium heat. Place the strawberry rhubarb mixture and cook until the fruits have softened, and the mixture is becoming syrupy, about 5 - 7 minutes. Take off the heat and set aside.

Preheat the oven to 350 degrees Fahrenheit.

Place the stick of butter in a 9” by 13” pan. Place in the oven so the butter can melt while the oven heats.

In the meantime, mix the flour, baking powder, 1/2 tsp salt, and 1 cup of granulated sugar. Add in the milk slowly, so the mixture doesn’t lump. Stir until you have a smooth batter.

Take the pan out of the oven. The butter should have melted completely by now.

Add the flour and milk mixture slowly. Do not stir.

Then slowly add the rhubarb and strawberry mixture. Spread it all around. The mixture will look quite soupy.

Place in the oven and bake for 30 to 45 minutes. Test it by using a toothpick and sticking in the sponge to see if it’s cooked through. Once it’s finished, take out of the oven and let cool.

Serve while its still warm with a dollop of heavy cream, some ice cream, or on its own. Cover and place any leftovers in the fridge for up to 5 days.