Bring a large pot of salted water to a boil and cook the pasta 2 minutes less than the package instructs. Drain and set aside.
Meanwhile, in a large nonstick skillet, cook the bacon over medium-high heat, stirring occasionally, until it begins to crisp, about 4 to 5 minutes. Add the garlic and cook, stirring, for 1 more minute. Remove the garlic and bacon and set aside.
In the same skillet, melt the butter over medium heat. Add the flour and cook, whisking, for 1 minute until a paste forms. Continuously whisking, add the milk in slow increments until incorporated into the flour paste. Cook until thick and smooth, about 8 to 10 minutes.
Add the Cheddar in 3 batches and stir, until it has melted and becomes a velvety sauce. Season with the salt and pepper. Add the macaroni, peas, and bacon-garlic mixture to the sauce.
Pour and scrape the macaroni into a 9-by-13-inch baking dish, sprinkle with the Parmesan, and bake until golden, about 20 minutes. Serve.