Combine flour and banana (a slightly unripe, still greenish banana is best) in a food processor, pulsing until there are no whole banana pieces left and you can mold the mixture (think play-dough). Transfer it out on to a clean surface and incorporate 1-3 tsp of warm water. You want enough water so that the dough is wet and not crumbly or dry, but not so wet it becomes sticky or hard to work with. Again, think "play dough". Roll it out using a glass (such as a drinking glass) until it's very thin. I find picking the dough up and moving it as I roll it out helps keep it from sticking to my clean surface. Once it is rolled out thinly, drape it over a greased, shallow 9" glass pie dish. Add your pie filling into the center, crumb topping over top and bake at 350F (regardless of what original pie recipe says). For a "premade crust" bake 10-15 minutes and let completely cool. If at any time the dough tears, push it back together with your fingers and a few drops of water.
To make apple pie, thinly slice 6-8 apples and toss with 1 tbsp cinnamon and 1/4 cup or more brown sugar. A few dashes of ground ginger is also a nice addition.