Preheat oven to 350 degrees and prepare your muffin tin with either liners or oil. Whisk together the flour, baking powder, baking soda, salt and pumpkin pie spice. (Old schoolers may prefer to sift the dry ingredients to be sure they are well blended, but I find whisking does the trick just fine).
With a mixer, combine the egg, sugar, yogurt, pumpkin, applesauce, and vanilla. Mix in the flour mixture until just combined. Stir in the walnuts*. Pour into the muffin tin and if you're using the crumble topping place a spoonful on each muffin. Bake for 20-25 minutes, or until a toothpick comes out clean.
*I have young children who can be finicky about nuts in their muffins, so I will often put my walnuts in the food processor and make them fine so the added nutrients are there, but it's hidden. If you do this, be sure to measure the nuts first, then process away!
Place walnuts in a food processor and pulse until finely chopped. Add the oats and give it a quick pulse or two. In a bowl, combine the flour, brown sugar, and walnut mixture. Cut in the butter until the mixture looks like coarse bread crumbs. Place on top before baking.