In a saucepan of salted boiling water, blanch the mint for 30 seconds. Transfer to a bowl of ice water; drain well and squeeze dry. Transfer the mint to a blender and puree with 3/4 cup of the olive oil and a pinch of salt.
In a bowl, toss the beans with the red onion, rosemary, marjoram, lemon zest and juice and remaining 2 tablespoons of oil. Season with the salt and pepper. Fold in the crab. Drizzle the salad with the mint oil and serve.
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