Saute the vegetables. Add olive oil and butter to a heavy bottomed soup pot. Sauté the onions, celery, and carrots over medium-high heat for 6 minutes. Stir in the garlic and cook for an additional minute, and then add the Italian seasoning, salt, pepper, and lemon zest. Stir in the flour and cook for 1 minute.
1 tablespoon olive oil, 1 tablespoon butter, 1 yellow onion, 2 stalks celery, 2 carrots, 3 cloves garlic, 1 tablespoon Italian seasoning, 1 teaspoon salt, 1/2 teaspoon pepper, 1 lemon, 2 tablespoons all purpose flour