Pat raw shrimp completely dry with paper towels. Toss in a mixing bowl with 1/2 tsp kosher salt, 1/4 tsp black pepper, garlic powder, and olive oil.
Heat 1 tablespoon of butter in a large deep cast-iron skillet over medium-high heat. Sear lemon slices for 1 to 2 minutes per side until charred, then set aside. Add seasoned shrimp in a single layer and sear for 1.5 to 2 minutes per side until pink and opaque. Transfer shrimp to a plate and keep warm.
Reduce heat to medium. Melt 2 tablespoons of butter in the flavorful pan drippings. Add minced garlic and drained jasmine rice. Toast the rice grains for 2 minutes, stirring constantly until fragrant and lightly golden.
Pour in chicken broth, Italian seasoning, lemon zest, red pepper flakes, and remaining 1 tsp salt, stirring thoroughly to scrape up browned bits from the pan bottom.
Bring to a rolling boil, then reduce heat to low, cover with a tight-fitting lid, and simmer undisturbed for 15 minutes until rice is tender and liquid is absorbed.
Remove lid and gently fluff the rice with a fork. Scatter green peas, sweet corn, and remaining 1 tablespoon of butter across the hot rice. Fold in seared shrimp along with any resting juices and drizzle with freshly squeezed lemon juice. Cover and let steam off the heat for 3 to 4 minutes.
Garnish generously with chopped parsley, cracked black pepper, and seared lemon rounds. Serve hot directly from the skillet.