In a small, dry skillet, toast the walnut pieces over medium heat, stirring frequently, until browned and fragrant, about 4 minutes. Reserve.
Heat the oil in a large, deep skillet over medium heat. Add the shallots and garlic and cook, stirring, until softened but not browned, about 2 minutes. Add the spinach in large handfuls, tossing constantly with tongs, until the spinach is wilted, about 2 minutes. Add the salt and pepper and toss. Take off the heat and add the reserved walnuts and feta. Drizzle with about 1 tablespoon of olive oil and serve.