In a small bowl or zip-top bag, combine the olive oil, balsamic vinegar, fresh mixed herbs, minced garlic, salt, black pepper, and red pepper flakes to create the marinade. Add the chicken breasts to the bag, ensuring they're well coated. Refrigerate for at least 3 hours (or up to 24 hours for deeper flavor—I find even 4-6 hours gives excellent results). While the chicken marinates, prepare your grill by heating it on high for 15 minutes until very hot, then lightly oil the grates with vegetable oil to prevent sticking.

While the chicken is grilling, combine the butter, 1 cup heavy cream, and whole milk in a heavy-bottomed saucepan over medium heat. Allow this mixture to simmer gently for about 20 minutes, stirring occasionally, until it reduces to approximately 3/4 cup—this concentration intensifies the cream's richness. I like to let the sauce reduce slowly rather than rushing it with high heat, as this creates a silkier, more luxurious texture without risk of curdling.

Remove the marinated chicken from the refrigerator and place it on the hot oiled grill grates. Grill over medium heat for 6-8 minutes without moving the chicken, allowing a golden crust to form. Flip the chicken and cook for an additional 5 minutes until the internal temperature reaches 165°F. Transfer the grilled chicken to a cutting board and loosely tent with foil, allowing it to rest for 15 minutes—this resting period ensures the juices redistribute throughout the meat, keeping it moist and tender.

Bring a large pot of salted water to a rolling boil and cook the fettuccine according to the package directions until al dente. Before draining, reserve 1/4 cup of the starchy pasta water—this liquid is essential for adjusting the sauce consistency. Drain the pasta and immediately return it to the empty pot off the heat.

Return the reduced cream sauce from Step 2 to medium heat and stir in the remaining 1/2 cup heavy cream, salt, and black pepper. Once the sauce is warm and well combined, immediately remove from heat and stir in the freshly grated Parmesan cheese and ground nutmeg, stirring constantly for about 2 minutes until the cheese melts completely and the sauce becomes silky and smooth. Add 2 tablespoons of the reserved pasta water to the sauce, then pour the entire Alfredo sauce over the cooked pasta in the pot. Gently toss the pasta and sauce together, adding more reserved pasta water as needed to reach your desired sauce consistency—I find the sauce should coat the noodles without being runny.

Slice the rested chicken breasts against the grain into strips or bite-sized pieces. Divide the creamy fettuccine alfredo among serving bowls or plates, top with the warm grilled chicken, and garnish generously with fresh chopped parsley and additional cracked black pepper. Serve immediately while the sauce is still warm and creamy.