Cabinet Meetings, or really this idea of The Cabinet as a President's inner circle of advisors.

All the Constitution says about them is this:

he may require the opinion, in writing, of the principal officer in each of the executive departments, upon any subject relating to the duties of their respective offices,

The President could go his entire time in office without ever once meeting one of his Secretaries in person if he wanted to.

But Washington gathered and consulted at length with Jefferson, Hamilton, and Knox, setting a precedent, and of course the recycling of the Founders and Framers brain trust for the next several administrations also helped lend the Cabinet weight.

It was also a hold over from British Parliamentary practice, though the break from an Executive formed by the legislature could easily have seen the idea fall by the way side.