Preheat the oven to 350°F (175°C). Lightly oil an ovenproof skillet and an 8x12-inch casserole dish.

Place tuna in the prepared skillet and bake until fish flakes easily with a fork, about 8 to 10 minutes.

Fill a large saucepan with lightly salted water and bring to a boil. Cook fettuccine until tender yet firm to the bite, about 8 minutes. Drain and set aside.

Remove tuna from the oven; leave the oven on.

Melt 3 tablespoons butter in a saucepan over medium heat. Cook onion and mushrooms, stirring occasionally, until softened, about 5 to 7 minutes.

Stir in flour until blended. Add milk, bouillon concentrate, adobo seasoning, and black pepper. Cook, stirring constantly, until slightly thickened, about 5 minutes. Remove from heat and stir in Parmesan cheese.

Melt remaining butter in the microwave (about 30 seconds). Stir bread crumbs into melted butter and set aside.

Add cooked fettuccine and flaked tuna to the cheese sauce and toss to combine.

Transfer noodle mixture to the prepared casserole dish. Top with torn bread.