Hi Terry! There's a WONDERFUL restaurant here in San Antonio called The Babylon, that serves a tuna steak on a bed of fresh, slightly wilted spinach with a vinagrette style dressing over the tuna and the spinache. It is soooooo good, and has a touch of dijon in it. THEY would not share the recipe, so I can't tell you what the dressing is. I'm wondering if the old standby glaze might not work for your tuna steak.
Don't know if this would work! The above is great on salmon done on a bbq grill. You combine brown sugar, mustard and soy sauce in med. bowl;whisk to blend. Transfer 1 tbsp. of glaze to a small bowl, mix in rice vinegar and set aside. Brush 1 side of salmon steaks (generously) with half of glaze in med. bowl. Place steak glazed side down, on bbq, grill until glaze is slightly charred, about 4 min. Brush top side of steak with remaining glaze in med. bowl.