Preheat the oven to 450 degrees. In a non-reactive bowl, combine the orange juice, balsamic vinegar, extra-virgin olive oil, onion, anchovy, orange zest and fresh herbs. Season with salt and pepper. (MAKE AHEAD: The dressing can be refrigerated for up to 2 days. Bring to room temperature before proceeding.) Place the spinach, fennel and radicchio in a heatproof bowl.

In a large non-reactive ovenproof skillet, heat the pure olive oil. Season the salmon fillets with salt and pepper and add them to the skillet, skinned side down. Cook over high heat for 1 minute. Transfer the skillet to the oven and roast the salmon until just cooked through, about 6 minutes. Remove to a plate.

Wipe out the skillet and add the reserved dressing. Bring to a simmer over high heat, then pour the hot dressing over the greens and toss to mix. Arrange the slightly wilted greens on 4 plates, set a piece o salmon on top and serve.