Heat oil in a large pot over medium heat. Add beets, apples, and salt. Cover, and cook, stirring occasionally, until partially tender, about 16 minutes. Uncover, and cook until liquid has mostly evaporated, about 4 minutes. Add 2 cups water, cover, and simmer over medium-high heat until completely tender and water has mostly evaporated, 20 to 25 minutes. Mash with a potato masher. Season with pepper.