In 10-inch skillet, melt butter over medium-high heat. Meanwhile, place cereal in food processor. Cover; process until mixture forms a coarse meal. Add cereal, salt and pepper to skillet. Cook 5 minutes, stirring frequently. Add walnuts to skillet. Cook and stir 3 to 4 minutes longer or until cereal is dark brown and walnuts are toasted slightly. Transfer to paper towels to cool.
In large bowl, mix lettuce, arugula, grapefruit sections, feta cheese and pomegranate seeds. In small bowl, mix poppy seed dressing, olive oil, vinegar and sugar. Stir until well combined. Drizzle dressing mixture over salad; toss to coat. Top with cereal mixture just before serving.