Preheat the oven to 350 degrees F (175 degrees C). Grease a 2-quart casserole dish.

Bring a pot of water to a boil. Add carrots and cook until tender but still firm, about 8 minutes; drain.

Melt 3 tablespoons butter in a medium saucepan. Sauté onion and stir in condensed soup, cheese, salt, and pepper. Stir in cooked carrots. Transfer mixture to prepared dish.

Toss croutons with 1/3 cup melted butter; scatter over casserole.

Bake in the preheated oven until heated through, 20 to 30 minutes.