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1
Preheat the oven to 350 degrees F (175 degrees C). Lightly oil an ovenproof skillet and an 8x12-inch casserole dish.
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2
Place tuna in the prepared skillet and bake in the preheated oven until fish flakes easily with a fork, 8 to 10 minutes.
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3
Fill a large saucepan with lightly salted water and bring to a rolling boil. Cook fettuccine at a boil until tender yet firm to the bite, about 8 minutes. Drain and set aside.
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4
Remove tuna from the oven; leave oven on.
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5
Meanwhile, melt 3 tablespoons butter in a saucepan over medium heat. Cook onion and mushrooms until softened, stirring occasionally, 5 to 7 minutes. Stir in flour until blended. Add milk, bouillon concentrate, adobo seasoning, and black pepper. Cook, stirring constantly, until mixture is slightly thickened, about 5 minutes more. Remove sauce from heat and stir in Parmesan cheese.
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6
Place remaining butter in a medium-sized microwave-safe bowl and heat in the microwave until melted, about 30 seconds. Stir bread crumbs into melted butter and set aside.
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7
Add cooked fettuccine and flaked tuna to the cheese sauce and toss to combine. Transfer noodle mixture to the prepared casserole dish and top with torn bread.
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8
Bake in the hot oven until golden brown, about 20 minutes.
