Imported stl oblject alignment to grid

I’m just finding my way as a new user. I have run into an issue I can’t seem to solve.
I imported a flat sided sphere (with other details already designed in) in stl format.

The flat side is at an odd angle compared to the XY grid. I want the flat side to lay down on the XY plane for 3D printing.

What is the best way to accomplish this?

I tried in object mode to add a plane and try to marry it to the bottom of the sphere, but I can’t seem to get it to snap exactly flat.

Any help would be greatly appreciated.

Here is how you can do it: