Cut 1/4 inch off the top of the bulb of garlic to expose the cloves beneath, but do not peel the garlic, you want to roast it in the skin. Place bulb on a piece of aluminum foil, place butter on top, pour lemon juice over the garlic and sprinkle with kosher salt. Bring up all sides of aluminum foil and twist together to close. Cook for 45 minutes or until garlic cloves are soft and golden in color. Allow to cool. Once cool, use a fork to remove the garlic cloves. Mash. Set aside.
Bring a large pot of water to boil. Cook pasta according to package directions. Drain.
Melt the butter in a large pot over medium heat. Whisk in the flour. Allow to cook for one minute. Slowly whisk in the milk. Heat to a simmer but do not boil.
Slowly whisk a couple tablespoons of the heated milk mixture into the egg to temper it. Pour into the milk mixture.
Whisk in the cheese and allow to melt. Whisk in the mashed roasted garlic, fresh thyme, black pepper and 1/2 teaspoon kosher salt. Stir in the pasta. Mix well. Remove from heat and let stand for 5 minutes so sauce can thicken.
Stir. Adjust seasoning if desired. Serve immediately.