In a medium saute pan over medium-high heat, melt 4 tablespoons of the butter. When the foam subsides, fry the cubes of bread until golden and crusty, about 4 to 7 minutes. Remove the croutons from the pan with a slotted spatula and set aside on paper towels to drain. Wipe out the pan.
Place the flour on a plate and season well with salt and pepper. Dredge the halibut in the flour and shake off excess.
Return the saute pan to the heat and melt 4 more tablespoons of butter. When the foam has subsided add the fish to the pan. Cook, turning once, until the fish is no longer translucent in the center, flakes easily, and is golden brown on both sides, about 4 to 7 minutes per side. Transfer the fish to a serving platter and wipe out the pan.
Melt the additional 2 tablespoons of butter, and when the foam subsides add the capers and croutons. Saute briefly, then add the lemon juice and parsley and swirl to combine. Pour over the fish on the serving dish and serve immediately.