Make the bark: Preheat the oven to 350. Combine the flour, oatmeal, walnuts, sugar and a pinch of salt in a bowl. Melt the butter in a pot over medium heat and cook until light brown, 6 minutes. Stir into the dry ingredients. Press the dough into the bottom of a 9-inch springform pan. Bake until golden brown, 30 minutes.
Remove the crust from the oven and sprinkle with the chocolate; let stand until soft, then spread with the back of a spoon. Refrigerate the bark until firm, 30 minutes. Break into pieces.
Make the oatmeal cream: Fill a large bowl with ice. Microwave 3/4 cup cream until simmering, 45 seconds. Puree with the oatmeal, sugar, a pinch of salt, vanilla and nutmeg in a blender. Transfer to a bowl and place over the ice; whisk until cool and thick. In another bowl, whisk the remaining 3/4 cup cream until soft peaks form; fold in the oatmeal puree.
Layer the oatmeal cream, bananas and crushed bark in 4 parfait glasses