Melt butter in a large skillet over medium heat. Add onion and garlic. Cook until onion is tender, about 5 minutes. Add chicken broth and cornstarch; mix until smooth. Cook until sauce is thickened, about 5 minutes more. Add 2 tablespoons Parmesan-Romano cheese blend, stirring until smooth. Add shrimp, sun-dried tomatoes, parsley, and lemon juice. Cook until shrimp turns pink, about 5 minutes more.
Bring a large pot of lightly salted water to a boil. Cook fettuccine in the boiling water until tender yet firm to the bite, 1 to 3 minutes; drain. Toss pasta with shrimp mixture. Garnish with remaining Parmesan and Romano cheese blend.