Add olive oil, onion, and garlic to a large soup pot and cook about 5 minutes over medium heat. Onion and garlic will become see-through.
While onion and garlic are cooking, chop celery. Add celery to the pot and continue to cook. As onion, garlic and celery are cooking, peel and cut carrots into half rounds. Add carrots to pot and continue to cook.
While other vegetables are cooking, peel and dice potatoes. Add to pot, along with lentils, basil, sauce, and broth/water.
Briefly stir to combine, then cover, turn heat to high, and bring to a boil. Once it reaches a boil, turn heat to low and simmer 30 minutes, stirring occasionally.
Toward the end of the simmer time, the potatoes will be soft and can be mashed a bit as you stir to thicken the stew.
After 30 minutes, stir in the frozen peas and allow them to heat through. Add salt, to taste.