Rethink rice. Wild rice, a semi-aquatic grass, is rich in antioxidants and minerals like magnesium, phosphorus and zinc.
Prepare rice according to package directions. Stir in ½ teaspoon each salt and pepper; cover and keep warm.
Sprinkle pork with remaining ¼ teaspoon each salt and pepper. Heat 2 teaspoons of the oil in a large nonstick skillet over medium-high heat. Add pork and cook, stirring occasionally, until browned on all sides, about 4 minutes. Remove pork from skillet. Add remaining 2 teaspoons oil to skillet; add mushrooms and cook, stirring occasionally, until browned, about 4 minutes. Add apple and sage; cook, stirring often, until tender, 3 to 4 minutes. Stir in spinach; cook, stirring occasionally, until just beginning to wilt, about 1½ minutes. Remove from heat. Return pork to skillet; stir to combine. Stir in cooked rice and sprinkle with walnuts and vinegar.