In a large pot or Dutch oven, heat 2 tablespoons of olive oil over medium heat. Add the chopped onions, green bell pepper, and red bell pepper, stirring frequently. Sauté for 8-10 minutes until softened and golden brown.

Increase the heat to medium-high, add the thinly sliced sirloin steak, and cook for about 4-5 minutes, stirring occasionally until browned.

Pour in 4 cups of beef broth and the diced tomatoes, stirring to combine, and bring to a gentle simmer for about 5 minutes.

Stir in the Worcestershire sauce, dried thyme, and optional red pepper flakes. Season with salt and black pepper to taste. Reduce heat to low, cover, and let simmer for 30-60 minutes.

In a separate saucepan, melt 4 tablespoons of butter over medium heat. Whisk in 1/4 cup of all-purpose flour and cook for 1-2 minutes until bubbling.

Gradually pour in 2 cups of milk while whisking continuously. Cook for another 3-5 minutes until thickened, then add cream cheese and both cheeses until melted and smooth.

Pour the cheese sauce into the simmering soup and stir gently to combine. Heat through over low heat for 5-7 minutes.

Adjust the soup's thickness by adding more broth if needed or simmer uncovered for an extra 5-10 minutes. Serve garnished with shredded cheese and parsley.