1. Heat 2 tablespoons oil in 12-inch nonstick skillet over high heat until beginning to smoke; stir in onion and cook until beginning to brown, about 1 minute. Add asparagus, 1/4 teaspoon salt, and 1/4 teaspoon pepper; cook until asparagus is browned and tender-crisp, about 4 minutes, stirring once every minute. Off heat, sitr in beans; transfer to large plate and cool 5 minutes.
2. Meanwhile, whisk remaining 3 tablespoons oil, vinegar, 1/4 teaspoon salt, and 1/8 teaspoon pepper in medium bowl until combined. In large bowl, toss arugula with 2 tablespoons dressing and divide among salad plates. Toss asparagus mixture with remaining dressing, place a portion over arugula, and serve.
STEP BY STEP: Cutting Asparagus on the Bias
1. To cut the asparagus quickly and evenly, arrange the spears in staggered groups of five on a cutting board.
2. Cut them into 1-inch pieces on the bias.