In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 3 - 4 minutes.
Add garlic and whisk to combine. Season with half of the salt and pepper.
Slowly whisk in milk and half & half. (Start slowly, to allow milk/half and half to blend in as you're whisking. Don't dump it all in at once)
Whisk frequently over medium/medium low heat until sauce has thickened.
When sauce has reached desired consistency, remove from heat and whisk in 1 - 1 1/4 cups grated Parmesan. Add additional salt and pepper to taste.
If sauce is too thick, add a splash of pasta water to thin it. Add a ladle or two of sauce to cooked pasta and toss to coat noodles with sauce. Add pasta to bowls for serving.
Gently fold thawed and drained langostino tails into remaining sauce and ladle sauce over pasta.
Top with fresh parsley or chives and remaining grated Parmesan cheese.