Adjust rack to center of oven and preheat to 400°F
Combine butter, capers, onion and lemon juice in food processor and pulse to blend. Set aside.
Place each halibut fillet on dry surface and sprinkle with salt.
Smear tops and sides of fillets with mustard and coat with bread crumbs and parsley.
Coat bottom of baking pan with half of the lemon-caper butter. Place fillets skin side down in pan and dot each fillet with remaining lemon-caper butter.
Bake until crusty and golden on the outside and moist and flaky in the middle, 15-20 minutes.
Carefully transfer baked fish to warm platter. Squeeze the lemons into the butter and drippins left of baking sheet and transfer to small bowl. Whisk together thoroughly and pour over fish. Serve immediately.