Preheat the oven to 425 degrees F. Line a muffin pan with paper liners or spray with a nonstick cooking spray.
For the muffin dry ingredients: in a large bowl stir together the whole wheat flour, baking soda, cinnamon, allspice, ginger, and salt.
For the muffin wet ingredients: in a separate medium bowl, mix together the ground flaxseed, water, applesauce, melted coconut oil, honey, and vanilla.
For the streusel topping: stir together all 7 ingredients (whole wheat flour, brown sugar, oats, walnuts, coconut oil, cinnamon and salt) with a fork until combined. Then use your hands to press and squeeze the crumbs together until they form clumps.
Spoon the muffin batter into the prepared muffin pan, filling each muffin cup just over 3/4 full.
Bake for another 15 minutes, or until a toothpick inserted into the muffin comes out clean (a crumb or two is fine, you just don't want wet batter stuck to the toothpick). The crumble topping will be golden brown.
Whole Wheat Apple Streusel Muffins https://thetoleranttummy.com/2018/07/20/whole-wheat-apple-streusel-muffins/