Skinny Alfredo

In a large pot of salted boiling water, cook linguine according to package directions until al dente. Set aside ½ cup of pasta water, then drain pasta and set aside. In a large skillet over medium heat, heat oil. Add garlic and cook until fragrant, 1 minute. Sprinkle flour over evenly, then stir and cook until mixture is lightly golden.  Very gradually add broth in while whisking, 2 tablespoons at a time, waiting for mixture to become completely smooth before adding more broth. Bring mixture to a boil, then gradually stream in milk while whisking. Bring to a simmer and cook until sauce is thickened, 2 to 3 minutes.  Remove from heat and add Parmesan and yogurt, if using. Season with salt, pepper, and a pinch of red pepper flakes.  Add pasta and a 1/4 cup reserved pasta water to sauce and toss to combine. If sauce is too thick add more pasta water, a tablespoon at a time, until desired consistency.  Garnish with parsley before serving.