Preheat the oven to 400 degrees. Cut the ends off of the sweet potatoes and reserve them. Place the washed sweet potatoes in a roasting pan lined with aluminum foil or just place them on a large piece of aluminum foil on the oven rack. Add the ends of the potatoes on the aluminum foil, too, as they are great for snacks, but watch them carefully so they do not burn.
The potatoes will need about an hour to cook and the bananas about 20 minutes, so just add the bananas toward the end of the cooking hour. Cook until both are very soft and squishy. Remove the potatoes and banana from the oven and allow to cool. If you are making this dish in advance, turn off the oven, but if not, leave it on.
When the potatoes are cool enough to handle, cut them in half lengthwise and scoop out the flesh into a large mixing bowl or food processor. The skin should really just come off quite easily. Alternatively, if you bought organic potatoes, consider tossing the skin in with the flesh--for some added nutrients. Slice the bananas in half lengthwise as well and add them to the bowl or food processor--juice and all.
Add 2 tablespoons of butter and a 1/4 cup of milk. Season the mixture with a pinch of kosher salt and process or beat vigorously with a wooden spoon or mash with a potato masher until everything is well combined and the mixture is fluffy. Taste it to make sure it tastes delicious. Spoon mixture into a round or square oven-proof baking dish and smooth the top.
In a separate bowl, use your fingers to rub together 3 tablespoons diced butter, brown sugar and flour until the mixture resembles coarse crumbs. Then add the chopped pecans. Alternatively, you could toss the butter pieces, flour, sugar and pecans in the food processor and pulse a few times until it all comes together.
Sprinkle the crumb mixture over the sweet potatoes and banana mixture and place in a 400 degree oven. Cook for about 20-30 minutes or until the crumbs are golden.
Serve immediately and watch it disappear.