To make the topping: Combine the flour, oats, brown sugar, cinnamon, salt and pecans in a large bowl until well-combined. Add in the pieces of butter and use your hands to squeeze and combine until the mixture becomes crumbly and resembles wet sand. (Alternatively, you can cut in the butter with a pastry cutter, or place all of the topping ingredients in a food processor and pulse until just blended. My favorite method is to use my hands, as I believe you get the best crumbly topping that way.)
TO MAKE GLUTEN FREE: in the topping, use certified gluten free oats and almond flour or an all purpose gluten free flour or gluten free oat flour.
TO MAKE VEGAN: sub a buttery vegan stick for the butter or use coconut oil.