Heat olive oil in a large stock pot over medium-high heat. Add onion, carrots, and celery and sauté for 5 to 7 minutes, stirring occasionally, until softened. Add garlic and crushed red pepper flakes and sauté for 2 minutes, stirring frequently.

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

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 just barely al dente. (Try to avoid overcooking the orzo!)

Stir the spinach into the soup. Taste and season with salt and black pepper, to taste.

Serve immediately, garnished with your favorite toppings, and enjoy!