Fill a large pot with water and bring to a boil. Add a small handful of salt to the water, followed by the tortellini. Boil tortellini until al dente, 4 to 5 minutes. Drain tortellini and set aside.
Place same pot back onto stovetop, over medium heat, and melt 1 tablespoon butter. Add sausage and brown, about 3 to 4 minutes. Remove sausage from pot and wipe clean.
Place pot back over medium heat and melt remaining butter. Add flour and whisk together until well combined.
Whisk stock and tomato sauce into flour mixture and continue to whisk together until no lumps remain. Season with salt and pepper.
Stir sausage and spinach into soup and simmer for 5 to 6 minutes.
Add tortellini into soup and stir together. Simmer soup for 5 minutes, adjust seasonings. Serve.