Submerge the paper-thin red onion slices in a small bowl of cold ice water for 10 minutes to remove their harsh raw bite. Drain thoroughly and pat dry with paper towels.

Place the chopped walnuts in a dry skillet over medium heat. Toast for 3 to 5 minutes, stirring frequently, until fragrant and lightly browned. Immediately transfer to a plate to cool.

Combine the olive oil, pomegranate molasses, fresh lemon juice, kosher salt, and black pepper (if using) in a small glass jar. Seal tightly and shake vigorously until smooth and emulsified. If the dressing feels too thick to pour, whisk in 1 teaspoon of warm water.

In a broad salad bowl, combine the dry baby spinach, mellowed red onion slices, cooled toasted walnuts, and pomegranate seeds.

Drizzle the vinaigrette over the salad right before serving. Toss gently with salad tongs, scooping up from the bottom so the nuts and pomegranate seeds get evenly distributed and coated.