Brush 1 tablespoon Dijon all over the flesh side of each piece of salmon, then sprinkle each with salt, pepper and 1 teaspoon herbes de Provence.
Heat a large nonstick pan over medium heat and add the olive oil. Place the salmon in the pan skin-side down and let it sizzle until the skin is golden brown and crispy, 6 to 7 minutes. Gently flip with a fish spatula. Add 1 tablespoon butter to the pan and let the salmon cook until the flesh is golden brown and the internal temperature is 130 degrees F, 2 to 3 minutes more. Add the remaining 1 tablespoon butter and the lemon juice, then remove from the heat. Serve the salmon on top of the Charred Cabbage Salad. Spoon the lemon pan sauce over the salmon and garnish with the crispy onions.
Charred Red Cabbage Salad:
Preheat the grill or a grill pan for cooking at high heat.
Drizzle the cabbage and onion liberally with olive oil. Sprinkle with salt and pepper. Grill the cabbage and onion until the cabbage is slightly wilted and super charred and the onion is soft and charred, about 5 minutes per side. Remove from the grill and set aside to cool until cool enough to handle. Remove the stems of the cabbage and roughly chop the cabbage; transfer to a medium bowl. Roughly chop the onion and add to the bowl along with the olives, red pepper, orange segments and juice, lemon juice, olive oil and salt and pepper. Toss to combine and adjust seasoning if necessary.