Using a pastry brush, lightly brush the parchment paper with some of the butter or ghee.
Make sure your cod is really dry and remove any visible bones using tweezers.
Place the cod fillets on the pan.
Fill in the rest of the pan with the asparagus, olives, capers, garlic, and tomatoes.
Brush the remaining butter onto the fillets and vegetables, making sure to cover the entire surface of the cod fillets.
Sprinkle everything with the salt and pepper.
Slice one lemon into rings and top each fillet with a lemon slice.
Place the sheet pan in the oven on the middle rack and roast for 10-12 minutes, until the fillets are opaque all the way through and flake easily with a fork. Cooking time will depend on the thickness of your fillets.
Zest the remaining lemon directly onto the dish before serving.