In a small bowl, whisk together the olive oil, miso paste, coconut aminos, rice vinegar, honey (if using), garlic, ginger, mustard powder, and 1 tablespoon water until smooth. Add a little more water if needed so the glaze is easy to brush onto the salmon. Set aside about one-third of the glaze for later.
Pat the salmon steaks dry with paper towels and season with sea salt and black pepper. Brush the remaining glaze over both sides.
Heat a cast iron skillet over medium heat. Lightly oil the skillet if needed. Add the salmon and cook for 3 to 4 minutes, until nicely browned. Carefully flip and cook for another 3 to 4 minutes.
During the last minute of cooking, brush the salmon with the reserved glaze.
If your salmon steaks are very thick, transfer the skillet to a 400°F oven for another 3 to 5 minutes, or until cooked to your liking.
