Vegan Thai-Spiced Sweet Potato Stew

1 tablespoon olive oil
1 medium onion, quartered and thinly sliced
4 to 6 garlic cloves, minced
3 medium sweet potatoes (about 1-1⁄2 pounds), peeled and diced
3 cups water
1 medium green or red bell pepper, cut into narrow strips
1-1⁄2 cups frozen green beans
1⁄2 teaspoon red or green curry paste, or to taste
1 tablespoon sugar
2 teaspoons minced fresh ginger
2 stalks lemongrass (optional)
1 (13.5-ounce) can light coconut milk
2 tablespoons peanut butter
Salt to taste
1 (8-ounce package) Thai peanut-flavored baked tofu, diced (optional)
Cilantro leaves for garnish