Combine all marinade ingredients into a small mixing bowl and whisk together. Place beef into a shallow baking dish and pour marinade over beef. Flip beef to coat the other side. Cover with plastic wrap and set aside for at least 30 minutes and up to an hour.
For stock: Pour stock into a large pot and add aromatics. Simmer for about 1 hour. Strain, season with salt and pepper and set aside.
Fill a large pot with water and place over high heat. Bring water to a boil. Add chuka soba noodles to the boiling water and boil for 1 minute. Remove from heat and cover for 5 minutes. Drain and set noodles aside.
Place a sauté pan over medium-high heat and add oil. Blot the meat dry with a paper towel and sear the pieces on each side for 2 to 3 minutes. Remove from heat and set aside.
Bring flavor infused stock to a boil and add spinach and kale an season with salt and pepper. Boil for 2 minutes, than add noodles. Add half of the beef and continue to boil for 2 to 3 minutes. Season with salt and pepper.
To serve: Dish out equal servings of the soup, top each with a few slices of the remaining beef and top with cilantro and chili flakes, if desired. Serve.