Slic3r settings for flat slopes on top
Photo Credit: Courtesy of 3DStockPhoto ( image)

What infill are you using? How flexible do you need the object to be?

I suggest two or three top and bottom layers, and a second or third solid perimeter as well.

The slicer has to determine if a particular path is part of an external perimeter or a top layer, so adding one to each should give a better result.

Thinner layers and a fan cooling the deposited material both help with overhangs like this.