1. Heat the oven to 350 degrees F. Melt 4 tablespoons butter in a small pot. Stir in the breadcrumbs, half the shallots, the parsley, lemon zest, and season with salt and pepper.
2. Sprinkle both sides of the halibut fillets with salt and pepper. Spread a thin layer of mayonnaise on top of each fillet, then press the panko mixture on top.
3. Heat the remaining 3 tablespoons butter in a 12-inch skillet. Add the rainbow chard stems and remaining shallots. Cook 2 minutes, then turn off the heat. Fold in the torn chard leaves, 2 tablespoons water, and salt and pepper, to taste. Place the halibut fillets on top of the chard leaves.
4. Transfer the skillet to the oven until the fish is just cooked through, 15 to 20 minutes.
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