Pat the sirloin steak pieces dry with paper towels. In a medium bowl, toss the steak with 2 tablespoons olive oil, 4 minced garlic cloves, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Ensure all pieces are well coated.

Heat a large stockpot or Dutch oven over medium-high heat. Add the seasoned steak in a single layer (cook in batches if necessary to avoid crowding the pan). Sear for 2-3 minutes per side until browned, but still slightly pink in the center, as it will continue to cook in the soup. Remove the steak from the pot and set aside.

Reduce the heat to medium. Add 1 tablespoon of olive oil to the same pot. Add the diced onion, carrots, and celery. Sauté for 5-7 minutes, scraping up any browned bits from the bottom of the pot, until the vegetables begin to soften.

Stir in the remaining 3 minced garlic cloves and cook for another minute until fragrant.

Pour in the beef broth and diced tomatoes (undrained). Add the Italian seasoning, red pepper flakes (if using), 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Bring the soup to a rolling boil.

Add the tortellini to the boiling soup. Cook according to package directions, usually 3-5 minutes for fresh tortellini or 5-7 minutes for frozen, until al dente.

Once the tortellini is cooked, stir in the fresh spinach. Cook for 1-2 minutes until the spinach has wilted. Return the seared steak pieces to the pot and stir to combine, heating through for about 1 minute.

Taste and adjust seasoning as needed. Ladle the hot soup into bowls. Garnish with fresh chopped parsley and grated Parmesan cheese before serving.