Heat a small pan over medium high heat and add oil. When hot, add the onions and cook, stirring frequently until golden brown, about 5-10 minutes. While the onion cooks, prepare the broth: Combine all ingredients in the slow cooker. Add the cooked onion then cover and cook for up to 10 hours, whatever works best for your timing. You can also boil this on low for an hour on the stove.
30-50 minutes before serving, freeze the steak to firm it up - this facilitates slicing. Once firm, cut against the grain into 1/8-inch thick slices. Keep in the refrigerator until serving after you cut the beef.
Run the kelp noodles under boiling water (I like to put mine into a fine mesh strainer in the sink and then bring a kettle of water to a boil, pouring several cups over the noodles. Then just shake them dry.)
When ready to serve, taste the broth, season with more coconut aminos, fish sauce, salt, and pepper - you want it to taste highly seasoned.
Place noodles and beef slices in a serving bowl. Add green onions, cilantro, and basil. Pour the steaming hot broth through a fine mesh strainer and over top of the serving bowl (it will cook the beef as it is sliced so thin). Serve immediately with lime wedges, soft boiled eggs, if using, sesame oil, and sriracha.