Cook the macaroni according to package instructions. Drain and set aside.
Using the same pot, melt the butter over medium heat. Once melted, whisk in the flour and cook the roux (butter-flour mixture) for 1-2 minutes. Then whisk in the milk and add the cream cheese. Bring the mixture to a simmer and whisk until thick and smooth.
Add the shredded cheese and whisk until smooth. Then add the cooked macaroni to the cheese sauce. Stir to coat, and salt and pepper to taste.
To serve: Break lettuce leaves off the head. Spoon the mac and cheese onto individual lettuce leaves and sprinkle with crumbled bacon and diced tomato. Pick up with your hands (like a lettuce wrap) and enjoy!