Cut sirloin steak into bite-sized pieces about 1 inch thick. Pat completely dry with paper towels. Season generously on all sides with salt, black pepper, and Italian seasoning. Let sit at room temperature for 10 minutes while you prepare other ingredients.

Heat large skillet over high heat until smoking hot. Add olive oil and let shimmer. Place steak pieces in single layer with space between each piece. Sear for 2 minutes without moving. Flip and cook another 1-2 minutes for medium rare. Remove to a plate and set aside.

While steak rests, bring large pot of salted water to a rolling boil. Add cheese tortellini and cook according to package directions (2-3 minutes for fresh, 3-4 minutes for frozen). Reserve 1 cup of pasta water before draining. Drain tortellini but do not rinse.

In the same skillet used for steak, reduce heat to medium. Add butter and let melt completely. Add minced garlic and sauté for 30 seconds until fragrant and golden. Do not let garlic burn or it will taste bitter.

Pour heavy cream into the skillet with garlic butter. Bring to a gentle simmer, stirring occasionally. Add grated parmesan cheese and stir constantly until melted and sauce is smooth. Season with salt, black pepper, and Italian seasoning. If sauce is too thick, add splash of reserved pasta water.

Add cooked tortellini to the garlic butter sauce. Toss gently to coat every piece in the creamy sauce. Add seared steak bites back to the skillet. Fold everything together carefully without breaking tortellini. Let warm through for 1-2 minutes.

Check sauce consistency and add more pasta water if needed to reach glossy, coating texture. Taste and adjust seasoning with more salt or pepper if needed. Remove from heat.

Transfer garlic steak tortellini to serving plates or bowl. Garnish with fresh chopped parsley and extra grated parmesan cheese. Serve immediately while hot and creamy.