In a medium bowl, combine the sliced sirloin steak with 2 minced garlic cloves, 1 tablespoon olive oil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Toss to coat evenly and set aside while you prepare other ingredients.

Bring a large pot of salted water to a rolling boil. Add the cheese tortellini and cook according to package directions, usually 3-5 minutes for fresh and 5-7 minutes for frozen, until al dente. Drain well and set aside.

Heat a large skillet or Dutch oven over medium-high heat. Add the marinated steak in a single layer, working in batches if necessary to avoid overcrowding. Sear for 1-2 minutes per side until nicely browned but still slightly pink in the center. Remove the seared steak from the skillet and set aside.

Reduce the heat to medium. Add 1 tablespoon of olive oil to the same skillet. Add the diced yellow onion and cook, stirring occasionally, until softened, about 3-4 minutes. Add the remaining 4 minced garlic cloves and red pepper flakes (if using) and cook for another minute until fragrant, being careful not to burn the garlic.

Pour in the crushed tomatoes and chicken or beef broth. Bring the mixture to a gentle simmer and cook for 5-7 minutes, allowing the sauce to thicken slightly and flavors to meld.

Stir in the heavy cream and 1/2 cup of grated Parmesan cheese. Continue to simmer for 2-3 minutes, stirring, until the sauce is creamy and heated through. Taste and adjust seasoning with salt and pepper as needed.

Add the fresh spinach to the sauce and cook, stirring, until it wilts, about 1-2 minutes. Then, gently fold in the cooked tortellini and seared steak bites. Toss everything together until thoroughly coated and heated through.

Serve immediately, garnished with extra grated Parmesan cheese and fresh chopped basil or parsley.