Preheat your oven to 375F. Wash the cauliflower head and trim the part of the stem that's protruding, as well as the leaves around the stem, so that the cauliflower can rest on a flat surface. Cut the cauliflower in half, and then cut two steaks from each side by simply slicing across the cauliflower. You'll have some florets left at the end--save these!
Heat a cast iron skillet or a large pan over medium high heat. Add half a tablespoon of oil to the pan. Place your first two steaks (or as many as you can fit) into the pan and sprinkle them with salt and pepper (add any herbs or spices you like). Sear for 3-4 minutes, or until the bottom of the steaks is browning. Flip the steaks, sprinkle the other side with salt and pepper, and sear the new bottom side for another 3-4 minutes. Transfer the steaks to a parchment or foil lined baking sheet and repeat this process with the other half tablespoon of oil and remaining two steaks.
Place the baking sheet with all four steaks in the oven and bake for 20-25 minutes, or until the steaks are tender all the way through. (If your skillet is wide enough to fit all four steaks, you can simply transfer it to the oven after searing.)
While the cauliflower steaks are baking, bring a medium pot of water to a boil. Add the edamame and whatever ends you have left from your head of cauliflower (I had about 1 heaping cup of florets). Boil for 3-5 minutes, or until the cauliflower is tender. Drain the edamame and cauliflower and transfer them to a food processor fitted with the S blade. Add the garlic, broth, lemon juice and zest, salt, pepper, and olive oil. Process the mixture for about a minute, stopping a few times to scrape the bowl down, until it forms a thick mash. It should be uniform and easy to scoop, but it should retain a little texture. Pulse in the tarragon and parsley. Taste the mash and adjust salt, pepper, and lemon to taste.
Divide the warm mash onto four plates (about 3/4 cup per plate). Layer the cooked cauliflower steak on top of the mash. Serve, with a drizzle of dressing if you like.
Leftover garlicky edamame mash will keep in an airtight container in the fridge for up to 3 days. Leftover cauliflower steaks will keep for up to 3 days, too. If they break apart while you store them, you can toss the florets into a salad!