From Cook's Illustrated Magazine
Arrange bread in one layer on large cookie sheet. Toast bread about 4 inches under broiler element until light golden brown, 3-4 minutes per side. Brush both sides with olive oil and rub with peeled garlic. Arrange on serving tray.
Process 2/3 of beans, oil, water, lemon juice, garlic, salt and pepper about 10 seconds, until smooth. Add remaining beans and rosemary; pulse just until incorporated, about five 1-second pulses.
Whisk together oil, vinegar, salt and pepper. Add onion and arugula and toss gently.
Divide bean spread evenly among bread slices. Top each slice with a forkful of arugula/onion mixture. Serve immediately.