Warm the olive oil and butter in a large saucepan over medium heat, then add the diced onion and cook for 5 minutes, stirring now and then. The onion should soften and turn translucent without browning heavily.

Stir in the garlic, crushed red pepper flakes, and flour, then cook for 1 minute while stirring well. The mixture should smell fragrant and slightly nutty, with no dry flour left behind.

Pour in the vegetable broth, diced tomatoes, and tomato sauce, then add the dried basil, Italian Seasoning, salt, and black pepper. The broth should look evenly blended and taste well seasoned before the pasta goes in.

Bring the soup to a gentle boil, add the cheese tortellini, and cook for a few minutes until tender. The tortellini should puff slightly and feel soft but still hold their shape.

Take the pan off the heat and stir in the spinach, parmesan cheese, and heavy cream. The spinach will wilt quickly, and the broth will turn lighter in color and noticeably creamier.

Stir in the chopped basil and serve right away. The finished soup should smell fresh and herby, with a creamy tomato broth that coats the tortellini nicely.