To make the sauce combine the gochujang chili paste, spicy brown mustard, rice vinegar, coconut aminos, and 1 tsp sesame seed oil. Mix well to incorporate all the ingredients. Set the mixture aside and place a heatproof silicone pastry brush next to the bowl, ready to go.
Go ahead and wash the Chinese broccoli well. Cut the Chinese broccoli in half, separating the stems from the leaves. Place them in separate piles.
Dry off the fish with paper towels to ensure the skin is nice and dry for a great sear. Season the skin side and the meat side of the fish with ½ tsp sea salt. Preheat a large nonstick skillet over medium-high heat until the pan is very hot. Add 2 tbsp avocado oil and swirl the pan to coat it. Add the fish skin side down.
Turn the heat down to medium heat. Allow the skin to cook on this side for several minutes, about 4-5 minutes on this lower heat to crisp up the skin. Flip the fish skin side up. Brush the skin liberally with the gochujang sauce. Cook meat side down for 1-2 minutes. Then flip the fish again and brush the sauce liberally on the meat side. Repeat this process until the skin is blackened but not burnt and the fish is cooked through. The fish is cooked properly when the fish is flaky and an internal temperature reaches 145 F.
While the fish is cooking, start to cook the Chinese broccoli. Heat another large nonstick skillet over medium-high heat. Add one tablespoon of avocado oil and swirl the pan. Add in the stem parts of the Chinese broccoli. Cook for 30 seconds, stirring constantly. Then add in ¼ cup of water and top the skillet with a lid. Allow cooking like this for 2-3 minutes. The stems should be bright green at this point.
Add in the leaves and toss them around with the stems. Place the lid on the skillet again and allow to cook for 4-5 minutes or longer depending on how crunchy you like the stems pieces to be. I like them to retain the crunch, and the timing here reflects that. Be sure to stir the stems and leaves occasionally to prevent burning. Season them with ½ tsp sea salt, the remaining sesame oil, and mix well.
Serve the hot fish with the cooked vegetables and enjoy!