thai-style beef and broccoli stir-fry

I called this thai-style because I used soy sauce, lime juice and peanuts. The beef I used was london broil, sliced thin against the grain. The only problem I had was that I let the meat cook for too long while I was chopping the peanuts, and it made it pretty tough. It was kind of a shame, because it was really nice and tender when I originally cut it. I'm wondering if the lime juice might have toughened it up some, as well.