To make these bowls, start by preheating your oven to 400°F. Make the marinade by whisking together the miso, tahini, rice vinegar, soy sauce, maple syrup, ginger, minced shallot, and black pepper in a large bowl.
Pour half of the mixture over the steak, coating it well, and set the rest aside — that reserved half is your sauce for serving.
Cut the zucchini into small pieces, about 1/4-inch thick, and spread them onto a large baking sheet. Spray lightly with avocado oil spray, sprinkle with salt and pepper, and toss with your hands to coat evenly.
Roast for 15 to 16 minutes, tossing halfway through, until tender and lightly golden.
While the zucchini roasts, spray a large nonstick skillet with avocado oil spray and set it over medium-high heat. Let the pan get very hot before adding the steak — you may need to work in batches depending on the size of your cut. Let it cook undisturbed for 3 to 4 minutes, then flip and cook for another 3 minutes undisturbed on the other side.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing. I like mine medium-rare, so cook it a bit longer if you prefer it more well done.
To assemble, divide the jasmine rice among four bowls and top with sliced steak, roasted zucchini, and pickled onions if using. Drizzle with the reserved marinade and serve immediately.
Leftovers keep well in an airtight container in the fridge for up to 3 days.