In a large pot of boiling salted water, cook pasta until al dente. Drain pasta and rinse under cold water. In a large bowl, toss pasta with beans and goat cheese.
Make dressing: In a bowl, whisk together oil, vinegar, and mustard and season with salt and pepper. (To store, refrigerate pasta mixture and dressing separately, up to 1 day.) To serve, toss pasta mixture with dressing, arugula, and onion.