• Preheat oven to 400°F. Line a large baking sheet with parchment paper.

Bread

• Place the Italian bread cut side up on the prepared baking sheet. Spread the softened butter evenly over both halves and sprinkle with garlic salt.
• Bake for 8 to 10 minutes, or until lightly golden and crisp around the edges. Remove from the oven and set aside.

Shrimp

• While the bread is baking, pat the shrimp dry with paper towels. Season evenly with kosher salt and black pepper.
• In a large skillet over medium-high heat, heat the olive oil until shimmering. Add the shrimp in a single layer. Cook for 1 to 2 minutes per side, just until golden and opaque on the outside but still slightly translucent in the center. Do not overcook.
• Transfer the shrimp to a plate. They will finish cooking in the oven.

Alfredo Sauce

• In a saucepan over medium heat, melt the butter. Add the diced onion and cook for 3 to 4 minutes, or until softened and translucent.
• Stir in the garlic and cook for about 30 seconds.
• Pour in the heavy whipping cream and bring to a gentle simmer. Do not boil.
• Stir in the Parmesan cheese, kosher salt, black pepper, and crushed red pepper flakes. Continue stirring until the cheese has melted and the sauce has thickened slightly, about 3 to 5 minutes. Remove from heat.

Assembly

• Spoon about ¾ cup of Alfredo sauce over each toasted bread half, spreading evenly to the edges. Reserve the remaining sauce for serving.
• Arrange the shrimp evenly over both halves of bread.
• Sprinkle mozzarella and white cheddar cheese over the shrimp.
• Return the bread to the oven and bake for 8 to 10 minutes, or until the cheese is melted and bubbly. For a more golden top, broil for 1 to 2 minutes, watching closely.
• Remove from the oven and let rest for 2 to 3 minutes.
• Garnish with chopped parsley. Slice and serve warm with the reserved Alfredo sauce on the side for dipping.