1Prepare an ice water bath by filling a large bowl halfway with ice and water; set aside.
2Bring a large pot of generously salted water to a boil. Add the shelled favas and boil until the bean inside the outer skin is bright green and firm but not hard, about 1 to 2 minutes. Drain the favas and immediately place in the ice water bath until cool. Peel the light green skin from each bean to reveal two bright green inner halves, discard the skins, and place the beans in a medium bowl.
3Heat 4 tablespoons of the olive oil in a large frying pan over medium heat until shimmering. Add the garlic, season with salt, and cook, stirring occasionally, until fragrant, about 30 seconds. Add the reserved favas and stir to coat with oil. Add the water, thyme, and rosemary and bring to a boil. Reduce the heat to low, cover, and simmer for 10 minutes. Remove the lid and continue to simmer, stirring occasionally, until the beans are tender, about 10 minutes more. (Add more water as needed, a tablespoon at a time, to keep the beans from sticking to the pan.)
4Remove and discard the thyme and rosemary sprigs. Transfer the fava mixture to a blender and blend on low until coarsely chopped. Transfer a third of the chopped fava mixture to a small bowl. Continue to blend until the remaining fava mixture is finely puréed. If the purée is too thick, add water a tablespoon at a time to reach the desired consistency. Transfer the purée to the bowl with the reserved chopped favas. Stir in the lemon juice and the remaining 2 tablespoons oil. Season with salt and pepper and drizzle with additional olive oil if desired. Serve warm or at room temperature.