Brown the sausage: Heat a large stockpot over medium-high heat.  Add the ground sausage and sauté until browned and cooked through, stirring and crumbling with a wooden spoon as it cooks.  Use a slotted spoon to transfer the cooked meat to a clean plate and set it aside, reserving 1 tablespoon of grease in the pot.  (If there is more grease in the pot, discard it.  If there is no grease remaining, add 1 tablespoon of olive oil to the stockpot.)

Sauté the veggies. Add the onion, carrots and celery and stir to combine.  Sauté for 6-7 minutes, stirring occasionally, until the onions are softened and translucent.  Add the garlic and crushed red pepper flakes and sauté for 2 more minutes, stirring occasionally.

Simmer the broth. Add the diced tomatoes, stock, Italian seasoning and cooked sausage and stir to combine.  Continue cooking until the broth reaches a simmer, then reduce heat to medium-low to maintain the simmer.

Cook the pasta. Stir in the orzo until combined.  Cover the pot with a lid and continue simmering the soup, stirring occasionally so that the orzo doesn't stick to the bottom of the pot, until the orzo is al dente.

Add the spinach and season.  Stir the spinach into the soup.  Taste and season with salt and black pepper, as needed.  (Feel free to also add in extra Italian seasoning, if you would like.)

Serve. Serve immediately, garnished with your favorite toppings.