Preheat the oven to 425 degrees . In a large bowl, whisk 2 tbsp. oil, the vinegar and Italian seasoning. Add the shallot, toss to coat and season with salt and pepper.
In a small bowl, toss the pistachios, panko, fennel and 1 tbsp. oil; season. Place the fish on a foil-lined baking sheet. Brush with the remaining 1 tbsp. oil; season. Divide the nut mixture among the fillets, pressing to adhere.
Bake until the fish just starts to flake in the center, 10 to 12 minutes.
Toss the mixed greens with the shallot mixture; season. Divide the salad among plates. Top each serving with a cod fillet.