Preheat the oven to 350°F (180°C or 160°C for a fan oven).
Line a 12 cup muffin pan with paper liners.
First, make the streusel-
Cut the flour into the butter using a pastry blender or two knives until it resembles coarse crumbs.
Add the sugar and continue to cut in.
Using a fork, stir in the cinnamon and the ¼ cup finely chopped pecans.
Now, for the muffins -
Beat the oil and the eggs together in a medium bowl or jug.
Beat in the vanilla and crème fraîche or sour cream.
Stir in the mashed bananas and set aside.
In another, larger bowl, mix together the flour, flaxseed, sugar, cinnamon, baking powder, baking soda, salt and ¾ cup chopped pecans.
Add the oil and egg mixture to the flour mixture in the larger bowl and stir until just combined.
Divide the batter between the muffin cups. (An ice cream scoop makes this easy.)
Sprinkle a bit of the streusel mixture over each muffin.
Bake for 20 to 25 minutes or until a skewer inserted in the centre of the muffins comes out clean. (It's a good idea to watch the muffins after 15 minutes - oven temperatures can vary so radically.)
Cool on a wire rack. Delicious warm or at room temperature. The muffins will freeze fairly well, although they definitely need to be reheated for a few minutes to make the streusel topping crunchy again.