In a large pot of boiling water, cook the soba noodles for 4-5 minutes until tender, then drain and rinse under cold water to prevent sticking.
Pat the sirloin steak dry with paper towels and cut into 1-inch cubes, then season evenly with the sea salt and black pepper.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering, then sear the steak bites for 2 minutes per side until a golden crust forms.
Whisk the honey, minced garlic, and tamari in a small bowl, pour it over the steak in the skillet, and toss for 30 seconds until the meat is glossy and well-coated before transferring to a plate.
Reduce the skillet heat to low and add the gochujang, Greek yogurt, and sesame oil, whisking constantly with a tablespoon of water to create a smooth, creamy sauce.
Fold the fresh spinach into the sauce until just wilted, then add the cooked noodles and toss thoroughly to ensure every strand is coated in the spicy cream.
Divide the creamy noodles into bowls and top with the honey garlic steak bites, drizzling any remaining pan sauce over the top.