Heat the oil in a dutch oven over medium-high heat. Add the onions and sauté them for 4-5 minutes until they get soft. Add the carrots, celery, and roasted garlic and continue to sauté for an additional 3 minutes. Add the stock, along with the tomatoes, pesto, orzo, thyme, Italian seasoning, and oregano, stir to combine. Reduce the heat to medium and let the orzo pasta cook to al-dente, about 10 minutes, stir occasionally.