In a small saucepan, cover the peanuts with oil. Set over medium heat and cook, slowly bringing the oil up to 350°F (175°C) and then holding it there, until the peanuts are golden, about 10 minutes. Using a slotted spoon, transfer the peanuts to a paper towel-lined baking sheet to drain, then transfer to a large metal bowl. Toss the peanuts with the salt and let cool.
In a large bowl, prepare an ice bath. In a large pot of salted boiling water, blanch spinach until just tender and cooked through, about 30 seconds. Using a spider, quickly transfer spinach to ice bath, stirring to chill rapidly. Drain spinach, then wrap in a clean kitchen towel and squeeze to express any excess water. Cut spinach into 2-inch lengths.
In a small bowl, whisk together soy sauce, seasoning oil, vinegar, and sugar until well combined.
When ready to serve, combine spinach, peanuts, minced garlic, and vinaigrette. Serve cold.
