Preheat oven to 400 degrees and prepare a 12 muffin pan by greasing or lining with paper muffin liners.
Make streusel by combining all ingredients together and mixing well with a fork or small whisk. Set streusel aside.
Get out two bowls. In one, mix together all of the dry ingredients from the flour to salt. In the second bowl mix together wet ingredients until there are no clumps. You may use a immersion blender if you want it to be really smooth but that's not needed. Make a well in the dry ingredients and slowly add in the wet ingredients using a fork to combine the wet into the dry. Mix just until incorporated.
Divide out batter into the prepared muffin pan. Top with streusel (if you don't use all of the streusel you can save the rest in your refrigerator for another batch of muffins).
Bake for 15-17 minutes or until a toothpick comes out clean. Cool in the pan for 5 minutes then transfer to a wire rack to cool completely.
If you can't find walnut oil feel free to sub the oil with melted coconut oil or canola oil.