Bring a large pot of salted water to a boil and cook the fettuccine according to package directions until al dente. Drain and keep warm.
In a large skillet over medium heat, add 1 tablespoon olive oil and 1 tablespoon butter. When the butter is melted and hot, add the shrimp seasoned with 1/4 teaspoon salt and 1/8 teaspoon black pepper. Cook about 2 to 3 minutes per side, until shrimp are pink and opaque. Transfer shrimp to a plate and set aside.
Add 3 tablespoons butter to the same skillet and melt. Add the minced garlic and chopped onion and sauté for about 2 to 3 minutes, until fragrant and the onion is translucent.
Add the sliced baby bella mushrooms to the skillet and cook until they soften and release their juices, about 4 to 5 minutes.
Pour in 1/2 cup chicken or vegetable stock and bring the mixture to a simmer. Return the cooked shrimp to the skillet and simmer for 1 minute to combine flavors.
Add the remaining butter to the skillet and stir until melted. Remove the skillet from the heat and season with an additional 1/2 teaspoon salt and 1/4 teaspoon black pepper, to taste.
Toss the cooked fettuccine into the skillet until evenly coated with the sauce. Serve immediately.
