Cook the Onions, Carrot & Celery: In a large pot or Dutch oven, heat oil over medium heat until shimmering. Add onion, carrot, celery, 1/4 teaspoon salt, and 1/2 teaspoon pepper. Cook, stirring occasionally, until the vegetables are softened and fragrant, about 8-10 minutes. Add garlic and cook for 1 minute, until aromatic. Stir in split peas, making sure they're well coated with the oil and vegetables.