Heat oven to 350 F.
Make the topping first. In your 10-inch cast iron skillet, melt the butter over medium heat, add the brown sugar, lemon juice, and salt, and whisk until the brown sugar melts… one or two minutes.
Keep cooking for another minute or two until it starts to smell like caramel. Don’t take it too far or your caramel will burn. Turn off the heat, add the bananas and nuts, coat them carefully in the goo, and then arrange it so that it looks nice enough to be the top of your cake when you flip it out.
Make the cake: whisk together the melted butter, sugar, and vanilla until thoroughly combined. Whisk in the sour cream or yogurt, then work in the baking powder, baking soda, kosher salt, and cinnamon. Add the flour and using a rubber spatula, gently fold in without over working the batter.
Pour the batter over the fruit in the pan, using an off-set spatula to spread it to the edges without disturbing the fruit too much. Pop into the oven and bake for 35 to 45 minutes until it’s golden brown on top and a toothpick comes out clean.
Use a paring knife to cut around the edges right away to make sure it doesn’t stick. Let cool for 15 minutes and then, very carefully, place your cake stand upside-down on top of the pan and flip both over so the skillet is face-down on top. Your cake should slide right out. If any bits remain behind, don’t freak out: use an off-set spatula to surgically reapply them to the cake.
Allow the cake to cool completely and serve by itself or, even better, with a scoop of vanilla ice cream.