Heat 2 tablespoons olive oil in a large skillet or Dutch oven. Peel sweet potatoes or yams and chop into 1-inch cubes. Sauté sweet potatoes or yams until they just begin to soften. Remove from heat and transfer to separate bowl.
Heat remaining 2 tablespoons of olive oil in same skillet and sauté onion and garlic over medium heat until onions are golden brown. Add remaining ingredients, cover and simmer for 15 minutes. Add sweet potatoes or yams and continue to simmer until vegetables are tender (10-15 minutes). Add salt and pepper to taste.