In a large (5.5 quart or larger) heavy-bottomed pot, over medium heat, add olive oil.
Add onion, carrot, celery, and salt/pepper to taste (I add 2 teaspoons salt and 1 teaspoon pepper). Cook until vegetables are soft and onion is beginning to turn golden, 7–9 minutes.
Add garlic, Italian seasoning, cumin, thyme, red pepper flakes, and cook, stirring constantly, 1 minute.
Add in rinsed split peas, bay leaves, and vegetable stock. Stir, bring to a boil, then reduce heat to low and simmer uncovered, 60–90 minutes, stirring occasionally so peas don’t catch on the bottom of the pot.
Remove and discard bay leaves. Remove 2 cups soup and blend until completely smooth. Return blended puree to the soup and mix through.
If using, zest and juice lemon to get 1 teaspoon zest and 2 tablespoons juice; add to soup.
Taste and season, adding additional salt/pepper as needed.
Ladle soup into bowls and serve with hearty buttered bread, a sprinkle of pepper, and fresh parsley or thyme if using. Add grated Parmesan cheese to individual bowls if desired.
