Heat oil in Dutch oven over medium-high heat until shimmering. Add sweet potato, spread in an even layer, and cook without stirring until gold brown, about 5 minutes. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper and continue to cook, stirring occasionally, until potato is tender and browned, about 5 minutes. Transfer to a bowl.
Melt 1 tablespoon butter in now-empty Dutch oven over medium heat. Add asparagus spears, a pinch of salt, and a pinch of pepper. Cook, stirring occasionally, until asparagus is crisp tender, 4 to 6 minutes. Transfer to a bowl.
Meanwhile, place broth and water in a large (2 1/2 qt) saucepan. Cover pot, bring to a boil over high heat, then reduce heat to low to keep warm.
Melt 3 tablespoons butter in now-empty Dutch oven over medium heat. Add onions, 1/2 teaspoon salt, and 1/2 teaspoon pepper and cook until onions are softened, 5 to 7 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in rice and cook, stirring often, until grain edges begin to turn translucent, about 3 minutes.
Stir in wine and cook, stirring constantly, until fully absorbed, 2 to 3 minutes. Stir in 5 cups hot broth mixture and half of browned sweet potato cubes. Reduce heat to medium-low, cover, and simmer until almost all liquid has been absorbed and rice is just al dente, 18-19 minutes, stirring twice during cooking. (There may still be a layer of liquid on the top until you stir - that's normal. Don't dry it out!)
Add 3/4 cup hot broth mixture and stir gently and constantly until risotto becomes creamy, about 3 minutes. Stir in Parmesan. Remove pot from heat, cover, and let sit for 5 minutes. Stir in lemon juice and remaining 1 tablespoon butter. Season with salt and pepper to taste and gently fold in remaining browned sweet potato cubes, asparagus, and chicken, if using. Before serving, stir in remaining broth mixture if needed to loosen consistency of risotto. Sprinkle individual bowls with pine nuts, fresh thyme and extra Parmesan, if desired.