1. Adjust oven rack to lower-middle position and heat oven to 325 degrees. Season beef with salt and pepper. Heat oil in Dutch oven over medium-high heat until shimmering. Add onions and ¼ teaspoon salt and cook, stirring occasionally, until well browned, 8 to 10 minutes.
2. Add tomato paste and garlic and cook until rust-colored and fragrant, about 2 minutes. Stir in flour and cook for 1 minute. Whisk in broth, ¾ cup Guinness, sugar, and thyme, scraping up any browned bits. Bring to simmer and cook until slightly thickened, about 3 minutes. Stir in beef and return to simmer. Transfer to oven and cook, uncovered, for 90 minutes, stirring halfway through cooking.
3. Stir in potatoes and carrots and continue cooking until beef and vegetables are tender, about 1 hour, stirring halfway through cooking. Stir in remaining ½ cup Guinness and parsley. Season with salt and pepper to taste, and serve.