Bring a large pot of salted water to a rolling boil. Cook fettuccine pasta according to package directions until al dente (about 9 to 10 minutes). Reserve 1/2 cup of starchy pasta water, then drain pasta.

Pat shrimp completely dry with paper towels and season with kosher salt, black pepper, and crushed red pepper flakes.

Heat 2 tbsp butter and 1 tbsp olive oil in a large deep skillet over medium-high heat. Add shrimp in a single layer and sear for 1.5 to 2 minutes per side until pink, opaque, and lightly golden. Transfer shrimp to a warm plate immediately.

Reduce skillet heat to medium. Add remaining 2 tbsp butter and minced garlic; sauté for 60 seconds until fragrant without browning. Pour in chicken bone broth and lemon juice to deglaze, scraping up all flavorful browned bits from the pan.

Pour in heavy cream and bring to a gentle simmer for 2 to 3 minutes until slightly thickened. Turn heat to low and whisk in grated Parmigiano-Reggiano until silky and smooth.

Add drained fettuccine and seared shrimp back into the skillet. Toss with tongs for 1 minute until pasta is glossy and thoroughly coated in the sauce (splash in reserved pasta water if needed to loosen).

Remove from heat, garnish generously with fresh chopped parsley and extra grated parmesan, and serve immediately!