Marinate steaks: Finely grate 2 large garlic cloves. In a small baking dish, whisk together garlic and all but 1 teaspoon tamari (save rest for step 5). Pound steaks to an even thickness if necessary, add to marinade and turn to coat, poking steaks with a fork a few times on both sides to help it absorb. Set aside at room temperature to marinate.
Cook potatoes: Scrub potatoes, then cut into 1-inch pieces (no need to peel); place in a medium pot with 2 teaspoons salt. Add cold water to cover by 1-inch. Cover, bring to a boil, then cook, uncovered, until potatoes are just tender, 6-8 minutes. Drain well.
Steam snow peas: Trim stem ends from snow peas, then thinly slice lengthwise. Return potatoes to pot and cook over medium-high to remove moisture, stirring gently, about 1 minute. Remove from heat, then immediately place snow peas on top to steam. Set aside. Trim ends from scallions, then thinly slice on an angle (about 1⁄2 cup).
Cook steaks: Heat a grill or grill pan over high and lightly brush grates with oil. Grill steaks, occasionally brushing with marinade, until browned and caramelized on the outside and pink in the middle, 3-4 minutes per side for medium-rare. Transfer to cutting board; allow to rest for 5 minutes.
Dress potato salad: In a large bowl, whisk rice vinegar, remaining tamari, 3 tablespoons oil, 1⁄4 teaspoon sugar, 3⁄4 teaspoon salt, and a few generous grinds pepper. Add potatoes, snow peas, and scallions to dressing and gently toss.
Toast sesame & serve: Place sesame seeds in small skillet and toast on medium-high heat, stirring constantly, until golden brown and fragrant, about 3 minutes. Stir into potato salad and season to taste with salt and pepper. Serve steak topped with any accumulated juices from cutting board, and with potato salad alongside. Enjoy!