Whenever I'm looking for a side dish to pair with steak, my mind tends to gravitate toward mushrooms, since the two are such a fantastic pair. The only problem is a visual one: both are brown, making for a monochromatic dish. Steakhouses solve this problem by adding a completely unnecessary sprig of parsley, but I decided to go all in and add as much green as possible. That's how I ended up with the idea of a pesto.
Well, at least for a while. The sauce started out as a chunky, oily pesto, albeit one made with pumpkin seeds and cilantro instead of pine nuts and basil. But I also wanted it to have some zing, so I squeezed some lime juice in. Turns out, the more lime juice I added in the more I liked it, making this more like a salsa than anything else (without any chilies, of course). Since the pumpkin seeds are toasted first, they have a smoky, roasted aroma, which is cut by the freshness of the cilantro and lime. It's great stuff.
Because the sauce is so flavorful, I didn't want to do anything to the steak except season it well and cook it to medium-rare. While grilling would be a fantastic option for the summer, that's not a convenient weekday option for me. Instead, I just cooked it in my heavy cast-iron skillet for about eight minutes. Just remember to slice the flank steak against the grain.
I took the same approach with the mushrooms, simply tossing them in the skillet once the steak was done. Once they release their water, you can scrape up any browned bits leftover from the steak, so the 'shrooms come out meaty, smoky, and tender.