Very simple. A "flat landscape" is a plain. Elevated "flat" landscapes are mesas and plateaus, but in this case the landscape is clearly lower than the hill, so it is a hill within or adjacent to a plain.
I believe that "plain" is the word for which you were looking. "Mesa" or "plateau" implies an elevated but flat landscape. "Plain," on the other hand, implies that the area being described is lower than its surroundings. Examples might be "it was a plain surrounded by mountains" or "there was a small hill overlooking the plain." I cannot think of an example where you would use "plain" to describe flat terrain that was higher than its surroundings.