Preheat the oven to 350℉
Toss the rhubarb, strawberries and lemon juice in a large mixing bowl and set it aside
Heat ¼ cup of brown sugar and 2 tablespoons of butter in a saucepan on the stove over medium heat, stirring constantly until the butter has melted and the mixture starts to bubble, about 2-3 minutes. Set it aside and let it cool for 5 mins.
Coat the sides of a round 8’inch pan lightly with cooking spray. Pour your sugar mixture into the pan then top it with the strawberry-rhubarb mixture and set it aside
Whisk the flour, baking powder, baking soda and salt in a medium mixing bowl and set it aside. In another medium bowl or the bowl of a mixer, beat the remaining brown sugar, remaining butter and oil on medium-high speed until fluffy, about 1 minute.
Beat in the eggs then add the yogurt/sour cream and vanilla and beat until the mixture is smooth. Add the flour mixture while mixing on low speed just until it is incorporated and the batter has formed. Spread the batter over the fruit.
Bake the cake until it pulls away from the sides slightly and the top is golden brown, 35 to 40 minutes. Let it cool in the pan on a wire rack for 10 minutes. Run a knife around the edges and carefully invert the cake onto a serving platter.