Prepare the tempeh meatballs as instructed. This can be done up to three days in advance of making the Swedish meatballs. You can also freeze prepared tempeh meatballs for up to six weeks.
Transfer the broth, vegan sour cream, soy sauce, paprika, flour, mustard, and nutmeg to a standing blender and blend till smooth.
Transfer the sauce to a large, deep skillet. Bring the sauce to a simmer over medium low heat. Cook, stirring constantly, for 3-4 minutes, or until the sauce has thickened enough to lightly coat the back of a spoon. Taste the sauce. Add freshly ground black pepper to taste and adjust the soy sauce as needed.
Add the tempeh meatballs to the skillet. Use a large, heatproof spoon to gently turn and stir the meatballs in the sauce, coating them all evenly.
Once the meatballs are fully coated and submerged in the sauce and warmed through, serve them over mashed potatoes or noodles. Garnish with fresh, chopped parsley leaves, if desired.