A very easy soft hackle pattern that I like to fish during the Hendrickson hatches both prior to, during and while spinners are falling. This particular fly can be altered in size and color for any mayfly hatch not just the Hendrickson Mayfly, so experiment and find what works for you.
Duns are sometimes the ticket to matching mayfly hatches, and there are many variations out there in terms of flies to represent this particular stage of the emergence. Well, here is an ultra simple one that just plain catches fish. Use this particular one for a Blue winged Olive hatch on your favorite stream or alter the color and size to imitate any Mayfly Dun. Good luck