Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the fettuccine, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 8 minutes.
Drain well in a colander set in the sink.
Melt 1/2 cup of butter in a large skillet over medium heat, and cook and stir the green onions, mushrooms, and andouille sausage until the mushrooms begin to release their liquid, about 5 minutes. Stir in garlic and tomato, and cook, stirring often, until the garlic is fragrant, 3 minutes; stir in the shrimp, and cook and stir until the shrimp are opaque and pink, about 3 more minutes.
Pour in white wine and lemon juice, and cook until the liquid has reduced by half, about 8 minutes.
Pour in heavy cream, and cook until the cream has reduced and thickened. Stir in the green, red, and yellow bell pepper.
Drop in about 2 tablespoons of the diced cold butter. Holding the skillet by the handle, swirl the sauce over medium-low heat until the butter has melted and incorporated; repeat several times with 2 more tablespoons of butter until all remaining butter is incorporated.
Remove the sauce from the heat, and stir in parsley. Season to taste with salt and black pepper. Gently toss the sauce with the cooked fettuccine to serve.
