Combine dry ingredients together in a large bowl. Stir in the wet ingredients, just until blended. Fold in the rhubarb and walnuts. Drop in to muffin cups {I like to use a 3 tablespoon cookie scoop} so the muffin cups are 2/3 full. Combine topping ingredients together in a small bowl and sprinkle over muffins. Bake for 18 to 22 minutes.