In a large heavy pot, heat the oil over medium high heat until hot. Stir in the onions, garlic, 3/4 teaspoon salt, and 1/2 teaspoon pepper into and cook, stirring occasionally, until golden, 6 to 8 minutes. Peel and dice the potatoes. Stir in the carrots, stock, potatoes, and thyme, then bring to a gentle boil. Cook, stirring occasionally, until the potatoes are very soft and starting to fall apart, 45 minutes to 1 hour. Season the soup with salt and pepper to taste and serve.