1. Heat oil in a medium saute pan over medium heat until hot but not smoking. Add onion, and cook, stirring occasionally, until soft, about 4 minutes. Add cabbage, 1/2 teaspoon salt, vinegar, and 1/2 cup water to pan; raise heat to medium-high. Cook, stirring, until cabbage begins to wilt and liquid has evaporated, about 8 minutes. Transfer cabbage to a bowl; cover with foil to keep warm.

2. Lightly coat a large nonstick skillet with cooking spray; place over medium heat. Season skinless sides of fillets with pepper and remaining 1/4 teaspoon salt. Place fillets, skin-sides down, in hot skillet. Cook, without turning, until skin is very crisp and golden brown, 5 to 6 minutes. Turn fillets; cook until flesh is opaque, about 3 minutes. Transfer fillets to a plate, and loosely cover with foil. Keep skillet on stove with heat off.

3. Wipe skillet clean with a paper towel; place over medium-high heat. Add mustard and 1/2 cup water to skillet; when the water begins to bubble, whisk until smooth. Cook mixture until thickened, about 2 minutes.

4. Divide cabbage among four plates, and top with fillets. Drizzle mustard sauce over fillets. Season with pepper, and serve immediately.