Pulse the bread in a food processor to get slightly coarse bread crumbs.
Coat a casserole dish just large enough to hold the fillets with 1 tablespoon butter. Place the fillets in the dish and season with salt, pepper, lemon zest and juice, and thyme leaves.
Melt the remaining 1/2 stick of butter in a large skillet with the smashed garlic. As soon as the butter starts to bubble, turn off the heat and let the garlic infuse the warm butter.
Remove the garlic chunks from the butter. Add the bread crumbs and lightly toss until all the butter has been absorbed. Turn the heat off, stir in the parsley and season, to taste, with salt and pepper.
Spread a layer of bread crumb mixture over the top of each fillet and bake for 12 to 15 minutes until bread crumbs are golden brown and fish is cooked through.