Bring a large pot of salted water to a boil. Add the elbow macaroni and cook until al dente, about 8–10 minutes. Drain and set aside.
While the pasta cooks, make the avocado sauce: combine the minced garlic, peeled and pitted avocados, lime juice, chopped cilantro, and a pinch of salt and pepper in a food processor or blender. Process until smooth and creamy; set aside.
Make the cheese sauce: melt the butter in a small saucepan over medium heat. Whisk in the flour to form a smooth paste, then slowly whisk in the milk until smooth. Cook, stirring, until the sauce begins to thicken.
Remove the saucepan from heat and stir in the shredded Pepper Jack until fully melted and the sauce is creamy.
Place the drained macaroni in a large bowl. Toss with the avocado sauce until well coated, then add the cheese sauce and stir until the pasta is evenly coated and creamy. Taste and adjust salt and pepper as needed.
Serve warm, garnished with fresh avocado chunks if desired.