Experts deeply disagree on a lot of things to do with steaks and their preparation, but there's one principle just about everyone accepts: The more it's marbled with fat, the better and tastier the steak will be. Which is great, unless you're counting calories or trying to cut down on your saturated fats. In that case, you might want to look at a leaner cut, like eye of round. It makes an appealing steak because it's compact and lean and comes in tidy little rounds, but unfortunately, it's also relatively tough compared to most grilling steaks. If you want to enjoy it, you'll need to tenderize it.
You'll often see round steaks sold as "marinating steaks." The idea behind that labeling is to let you know they're not tender enough to slap straight onto the grill, and that you'll be better off to marinate them for a while first to tenderize them. Unfortunately, although generations of cooks have been told this, marinades don't do a whole lot to tenderize a steak. Most marinades contain acids, which will indeed break down proteins, but only on the surface of the meat. The only marinades that do seem to tenderize with any reliability are dairy-based marinades, which mostly use buttermilk or yogurt as a starting point. Scientists aren't quite sure why these work as well as they do, but the generations of Southern cooks who soaked their chicken in buttermilk were definitely onto something.
Treat Them With Enzymes
One thing that absolutely will tenderize a steak, on its own or in a marinade, is an enzyme-based tenderizer. The two you'll see most often are papain and bromelain, both derived from tropical fruit. That's what's in the powdered meat tenderizers you see at the supermarket, and it really does work. The enzyme basically starts digesting the tight bonds between the steak's proteins, which makes it tender and easier to chew. The downside is that it can give your steak a weirdly mushy texture, which isn't especially appealing. It's one thing to know in your head that the enzyme digests proteins, but it's a whole other experience to put a piece of partially digested steak into your mouth. Overall, it's best to use tenderizing powders very lightly along with another method.
Beat Them With a Hammer
One of the best-proven ways to tenderize an eye of round, or any tough cut of meat, is with a meat mallet. Round steaks are tough because the bundles of muscle fiber are held together by very tight bonds, which are hard for your teeth to get through. Pummeling the steak with a mallet tears those bonds apart, leaving the steak slightly flattened and a whole lot easier to chew. It also opens up the fibers so marinades can flavor and tenderize them more effectively, or tenderizing enzymes can have better access to their molecular bonds. Alone or paired with another method, the meat mallet is one of your best options for tenderizing steaks.
If you love kitchen gadgets, you may see what's called a "jacquard" or "jaccard" tenderizer at your favorite store or website. It looks a lot like one of those self-inking stamps you use at the office, but instead of a rubber stamp, the handle pushes down a set of sharp, rigid needles to pierce the meat. Where a meat mallet flattens the steak and tears its fibers apart, a jaccard tenderizer cuts the fibers short. It has much the same effect as thin-slicing the meat, making it easier for your teeth to chew through it. The only thing about jaccard tenderizers is that they can carry bacteria from the surface of the meat to the interior, so you should cook your steaks as soon as possible after tenderizing them. You'll also want to disinfect it really, really well after each use.
Tenderize Them With Your Cooking Method
Round steaks aren't necessarily the best bet for grilling or pan-searing and serving in the traditional steak fashion. If you aren't absolutely committed to treating them that way, you can tenderize them pretty effectively by choosing a different cooking method. Slicing the steak thinly makes it perfectly chewable, for example, so it's perfect for stir-fries where you'll cut it into small strips. Alternatively, you can slow-cook it in stews, chile or dishes like Swiss steak, where it's simmered slowly in some kind of sauce. Round steak is leaner than the meats you'd usually choose for that kind of dish, so it won't be as moist as a piece of – for example – chuck, but it keeps down your calories and saturated fats, and the sauce will keep it from feeling unpleasantly dry in your mouth.