Deep dado cuts.

[Hans Hauge]

Sounds like, tracksaw, chisel, router. Thanks all.

[Tom Bussey]

I agree with Scott. but one can use the shank of the end mill as a rub coller I use end mills from MCS to cut stuff like you are asking. End mills have a longer shank than router bits. There are some router bits that have a solid piolet so it is not a new concept. Carbide is pricier but steel can tend to me more whippy in a long length. but the yours is not really that deep. I would rough the groove out and finish it to size and use the end mill to finish the sides and just a light cut on the bottom to flatten the groove