In a large soup pot over medium heat, add butter and olive oil. When heated, add onion and stir until onion starts to soften, about 3-5 minutes.
Add garlic, red pepper flakes, Italian seasoning, salt & pepper, and stir until fragrant, about a minute.
Add both cans of tomatoes, chicken stock, and tortellini. Bring to a boil, then lower heat to a gentle, bubbling simmer and cook until tortellini is done, about 10 minutes (depending on the brand you purchased).
When the tortellini is done, add heavy cream and spinach leaves. Stir until the spinach is wilted, the cream is heated, and soup has thickened a bit, about 5 minutes. Taste, and add more salt & pepper to taste, if necessary.
Serve with fresh, torn basil leaves and optionally you can add some grated Parmesan cheese.