Fill a large pot with water and bring to a rolling boil. Add salt to the boiling water, then add 10 ounces dry elbow macaroni. Stir and cook until al dente, about 8–10 minutes. Drain the pasta in a colander and set aside.
While the pasta is cooking, make the avocado sauce: place 2 cloves garlic (minced), 2 avocados (peeled and pitted), 2 tablespoons fresh lime juice, 1/3 cup chopped fresh cilantro, and salt and pepper to taste into a food processor or blender. Process until smooth and creamy. Transfer the avocado sauce to a bowl and set aside.
Make the cheese sauce: in a small saucepan over medium heat, melt 2 tablespoons butter. When the butter is melted, whisk in 2 tablespoons all-purpose flour to form a smooth paste.
Slowly whisk in 1 cup milk until the mixture is smooth. Continue stirring (use a wooden spoon or whisk) over medium heat until the sauce begins to thicken.
Add 2 cups shredded Pepper Jack cheese to the thickened sauce and stir until the cheese is fully melted and the sauce is creamy. Remove from heat.
Place the drained macaroni in a large bowl. Pour the avocado sauce over the macaroni and stir until the pasta is evenly coated.
Pour the cheese sauce over the avocado-coated macaroni and stir until the pasta is fully coated and creamy. Taste and adjust seasoning with additional salt and pepper, if desired.
Serve warm and garnish with fresh avocado chunks, if desired.