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, 9 to 11 minutes.
While the fettuccine is cooking, combine cornstarch and nutmeg together in a small bowl. Add chicken broth and whisk until smooth.
Melt butter in a large pan over medium heat. Add garlic and cook until fragrant, about 2 minutes. Pour chicken broth mixture into the pan, raise heat to medium-high, and bring to a simmer, whisking occasionally. Add 1/2 cup Parmigiano-Reggiano cheese; whisk until melted. Remove from heat and whisk in yogurt until sauce is smooth.
Drain fettuccine and place in a large bowl. Add Alfredo sauce and toss to coat. Season with salt and pepper. Top with remaining Parmigiano-Reggiano and serve.