Preheat a grill to medium-high (about 375°F) or preheat the broiler.
Oil the grill grates or, if broiling, line a baking sheet with foil and spray with nonstick spray.
Brush the salmon steaks with olive oil and season both sides with salt and freshly ground black pepper.
If broiling: place salmon skin-side down on the prepared sheet and broil 3–5 minutes, then flip and broil another 3–5 minutes until opaque and flaky.
If grilling: place salmon skin-side down on the oiled grates, cover, and cook 10–15 minutes until opaque in the center and flaky.
Once cooked, divide each salmon steak in half to create four portions and keep warm.
While fish cooks (or after), combine the cooked, drained rice noodles in a small pot over low heat with sesame oil, optional mustard oil, grated ginger, and coconut milk; stir gently until warm and evenly coated.
Taste the noodles and adjust seasoning with a little salt and pepper if needed, then transfer to serving plates.
Top the warm noodles with sliced radish, chopped mint, toasted sesame seeds, and a salmon portion per plate.