Crispy Skillet Salmon with Mustardy Cabbage

Name: Crispy Skillet Salmon with Mustardy Cabbage
Ingredients: 1 small yellow onion; 1/2 medium head green cabbage (1 to 1 1/2 pounds); 2 sprigs fresh rosemary; 1 tablespoon Dijon mustard; 1 tablespoon whole-grain mustard; 2 teaspoons honey or maple syrup; 1 tablespoon apple cider vinegar; 1 teaspoon low-sodium tamari or soy sauce; 4 (6-ounce) skin-on salmon fillets; 1 1/2 teaspoons kosher salt, divided; 1/2 teaspoons freshly ground black pepper, divided; 2 tablespoons olive oil; 2 tablespoons unsalted butter