To prepare the salmon, in a small bowl, combine mustard, lemon juice, thyme, rosemary, and pepper. Brush mixture all over both sides of salmon steaks and transfer salmon to a large, shallow baking dish. Cover with plastic wrap and refrigerate 30 minutes, or up to 4 hours. Preheat grill or broiler.
Place salmon steaks on grill or under broiler and cook 4 minutes per side, or until fork-tender and cooked through.
To make the salsa, place chile peppers on hot grill or under broiler and cook 4 minutes, turning frequently, until charred on all sides. Wrap peppers in plastic wrap and let stand 5 minutes. Remove skin from peppers, halve, seed, and cut into small dice-sized pieces. Transfer to a small bowl and add papaya and red onion. Set aside.
When ready to serve, spoon salsa over salmon steaks.