Preheat oven to 400 degrees. Bring a large pot of water to a boil with a large pinch of salt. Remove the steak from the refrigerator and allow to come to room temperature. Halve, core, and remove white ribs from the bell pepper, then thinly slice. Thinly slice the scallions, keeping the white and green parts separate. Peel and grate the carrot. Mince or grate the garlic.

Heat 1 tablespoon oil in a medium pan over medium-high heat. Season the steak with salt and pepper on both sides. Add the steak to the pan and cook 3-4 minutes per side, until deeply browned. Transfer the steak to a baking sheet and place in the oven to finish cooking to desired doneness, if necessary.

In the same pan you cooked the steak in, combine the soy sauce, stock concentrate, garlic, 1/4 cup water, and wasabi (to taste). Return the steak to the pan over medium heat. Then, tilt the pan towards you and begin spooning the soy mixture over the steak continuously for about 1 minute. Set the steak aside and reserve the soy mixture.

Add the soba noodles to the boiling water, reduce to a simmer, and cook for 6-7 minutes, until al dente. Reserve 1/4 cup pasta water, then drain.

While the soba noodles cook, halve and pit the avocado, then scoop the flesh into a large bowl. Halve the lime, then squeeze over the avocado. Mash thouroughly with a fork, until completely smooth, then season with salt and pepper.

Toss the soba noodles, bell pepper, grated carrots, scallion whites, and the remaining soy mixture into the avocado mixture. Season with salt and pepper. Toss to thoroughly coat. Add a splash of the reserved pasta water, if necessary, to thin out the dressing.

Thinly slice the steak against the grain and serve atop the soba noddle salad, sprinkled with scallion greens and sesame seeds.