Bring a large pot of salted water to a boil. Cook pasta shells to al dente according to package instructions (usually 8–10 minutes). Reserve 1/2 cup pasta water, then drain.

Pat steak cubes dry with paper towels; season with salt, pepper, and optional Cajun seasoning.

Heat olive oil and 1 tbsp butter in a large skillet over high heat until shimmering.

Add steak in a single layer (work in batches if needed) and sear 1–2 minutes per side until browned but not overcooked (internal temp should be about 125–130°F for medium-rare). Remove steak to a plate and tent with foil to rest.

Reduce heat to medium. Add remaining 2 tbsp butter to the skillet and melt.

Add minced garlic and sauté 30–45 seconds until fragrant—don’t let it brown or it will taste bitter.

Stir in garlic powder to deepen the garlic flavor.

Pour in broth and heavy cream; scrape any browned bits from the pan.

Bring to a gentle simmer, then add cream cheese and whisk until smooth.

Stir in grated Parmesan and optional mozzarella. If sauce thickens too much, loosen with reserved pasta water a tablespoon at a time.

Season sauce with salt and black pepper to taste.

Add drained shells to the sauce and toss to coat. Fold the rested steak into the pasta just to heat through.

Garnish with chopped parsley and serve immediately.