Melt butter in a saucepan or wok over medium heat. Add onion and cook until soft and translucent, about 5 minutes. Add garlic and cook until fragrant, about 1 minute. Add salmon and tomato and season with salt and pepper.
Increase heat to high and cook, stirring constantly, for 2 minutes. Pour in vodka, reduce heat, and simmer, until liquid is reduced by half, 3 to 5 minutes. Pour in cream and simmer until liquid reduced by half again, 3 to 5 minutes. Adjust seasoning with salt and pepper.
Meanwhile, fill a large pot 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, add to sauce and toss to coat.
Serve sprinkled with caviar and freshly grated Parmesan cheese. Garnish with chopped parsley.