Bring a large pot of salted water to a boil. Cook the pasta according to package directions for al dente, adding the asparagus to the pot during the last 3 minutes of cooking. Drain the pasta and asparagus and run under cold water to cool; drain well.
Meanwhile, put the onion in a medium bowl and cover with cold water; set aside for 10 minutes.
Whisk together the olive oil, lemon zest and juice and 1/2 cup of the feta in a large bowl until well combined; season with salt and pepper. Add the pasta and asparagus, beans, parsley, dill and remaining 1/2 cup feta. Drain the onions, pat dry and add to the bowl. Toss everything together and season with salt and pepper.
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