You've got a nice set-up for them. That sounds like a pretty good feed to me. I'm sure they added the sesame seed because it's one of the better plant sources of methionine and they're quite high in protein. So, I think they were going for a balanced amino acid profile. Did the pecking start right after you changed feed? If it started before the food change, just ignore the next two paragraphs.
Sometimes chickens can be fussy about feed changes. They don't like the flavor as much, they don't like pellets as much as crumble or it just looks different. Is this feed a crumble, pellet or one of the feeds that looks more like grain, with the powdery ingredients just loosely mixed in? I'm wondering if she may not like it as much.
Could she either be not eating enough and be hungry or just cranky about the change? The other thing I would wonder about is if she isn't eating as much and could be filling up on flock block. I don't remember how flock block compares to regular feed. Is it possible that it might not be fulfilling her needs, if she is doing that? If the new feed is the type of feed where a lot of the ingredients are a loose powder, she may not be getting enough of the powdered part. That would include the kelp, that provides salt, along with a lot of minerals and vitamins. Sometimes you can mix some of that with a little yogurt, to get them to eat it.
If she's always been this way, then it may just be that she's a really aggressive chicken. It's always good to review any underlying issues that could be contributing to pecking and fix them. Even if all the chickens aren't pecking, they could still be stressed by whatever is driving the more aggressive chicken. Chickens can peck for a lot of different reasons, including anything that makes them cranky. Even not getting enough sleep because a bright light is on all night or because external parasites are biting them. Sometimes more aggressive chickens don't mix well with more docile breeds. Sometimes it's because they're crowded. Sometimes it's because you changed their schedule and now their free range time is limited. Sometimes it's because they're missing something in their diet, like enough protein, a particular amino acid is low or they aren't getting enough salt. It could even be because they aren't getting enough food or water, in cases where they don't have free access all the time.
Every once in awhile, a person can just get a particular chicken that is unusually aggressive. You could cull, try keeping her separate long enough to lower her status in the flock, trim her upper beak so she can't get a good grip on her victim or try a pair of peepers on her, so she can't get a good targeting aim on her victim.