Heat the olive oil in a large skillet over medium-high heat. Add the scallions and cook, stirring occasionally, until softened slightly, 3-4 minutes. Add the vegetable broth and lemon juice, raise the heat reduce until 2 tablespoons of liquid remains, 4-5 minutes. Add the cream, bring to a boil, reduce the heat and simmer until a thick sauce is formed, 3-4 minutes. Season with salt and pepper. Reduce the heat to a low simmer, cover and keep warm.
Cook the tagliatelle according to the package directions and drain, reserving 1/2 cup of the pasta cooking water. Add add the salmon and lemon zest to the cream sauce, then add the tagliatelle and toss to coat, adding the pasta cooking water a little at a time to loosen the sauce if needed. Season with salt and pepper and divide among plates.