Easy, peasy: All you need to do is saute, steam, and mash.
Heat oil in a large pot over medium heat. Add peas, spinach, and salt. Cover, and cook, stirring occasionally, until spinach is wilted and peas are partially tender, about 4 minutes. Add 1/2 cup water, cover, and simmer over medium-high heat until completely tender and water has mostly evaporated, about 2 minutes. Mash with a potato masher. Season with pepper.
