Heat the remaining tablespoon of oil in a large nonstick skillet over medium-high heat. Add the shallots and cook, stirring occasionally, until beginning to brown, 2 to 3 minutes. Add the sweet potatoes and cook, tossing occasionally, until tender, 7 to 9 minutes more. Add the spinach and cook, tossing, until just wilted, 1 to 2 minutes more.