Bring a large pot of salted water to a boil. Cook 12 oz shell pasta until al dente according to package instructions. Reserve 1 cup of pasta cooking water before draining, then set pasta aside.
Pat steak cubes completely dry with paper towels. Toss with salt, black pepper, 1 tsp garlic powder, and 1 tsp paprika until evenly coated.
Heat 2 tbsp olive oil in a large skillet over medium-high heat until shimmering. Add steak cubes in a single layer without overcrowding. Sear for 2–3 minutes per side until deeply golden brown. Work in batches if necessary.
Reduce heat to medium. Add 4 tbsp butter and 5 minced garlic cloves to the skillet. Stir continuously for 30 seconds until fragrant. Return all steak bites to the pan, toss to coat in garlic butter, sprinkle with fresh parsley, and remove from heat.
In a separate saucepan, melt 2 tbsp butter over medium heat. Whisk in 2 tbsp flour and cook for 1 minute. Slowly whisk in 2 cups heavy cream and 1 cup milk. Simmer for 3–5 minutes until slightly thickened.
Gradually stir in 1½ cups freshly grated Parmesan until melted and silky. Season with salt, pepper, Italian seasoning, onion powder, and red pepper flakes. Fold in cooked pasta shells. Add reserved pasta water a splash at a time if the sauce becomes too thick.
Ladle creamy Parmesan shells into bowls, top with garlic butter steak bites, and drizzle extra garlic butter over top. Garnish with fresh parsley and extra Parmesan.
Storage & Reheating Instructions
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Refrigeration: Store leftovers in an airtight container in the refrigerator for up to 3 days.
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Reheating: Warm gently over medium-low heat on the stovetop (or in short microwave intervals) with a splash of milk or cream to restore sauce texture.
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