1. Bring 4 quarts water to boil in large pot over high heat. Using ladle, fill each of 4 individual serving bowls with about 1/2 cup boiling water. Set aside.
2. Heat butter over medium heat in large saucepan until foaming. Whisk in flour until mixture is smooth and golden, 1 to 2 minutes. Whisk in milk, half-and-half, garlic, 1/2 teaspoon salt, 1/2 teaspoon pepper, and nutmeg and bring to simmer. Reduce heat to medium-low and simmer until sauce is slightly thickened, 1 to 2 minutes. Discard garlic, stir in Parmesan, and remove from heat.
3. Stir 1 tablespoon salt and pasta into boiling water and cook, stirring constantly, until al dente, 2 to 3 minutes. Reserve 1 cup pasta cooking water, then drain pasta.
4. Return sauce to low heat, add 1/3 cup pasta water and cooked pasta; toss until evenly coated. Cook until sauce has thickened slightly, about 1 minute. Add more cooking water as needed to thin sauce to proper consistency and adjust seasonings. Working quickly, empty water from warmed bowls and fill with pasta. Serve immediately.