Preheat the oven to 350°F (175°C).
Cook the elbow macaroni according to the package instructions until al dente. Drain and set aside.
Melt the butter over medium heat in a large pot. Add the flour and whisk until it forms a smooth paste, creating a roux.
Slowly add milk while whisking continuously to avoid lumps from forming. Continue whisking until the sauce thickens.
Add the mustard powder, cayenne pepper (if desired), salt, and pepper to the sauce. Stir well to combine.
Reduce the heat to low and gradually add the shredded cheddar and Monterey Jack cheeses, stirring until melted and smooth.
Remove the pot from the heat and add the cooked macaroni to the cheese sauce. Stir until the macaroni is evenly coated.
Transfer the mixture to a greased baking dish, spreading it out evenly.
Place the dish in the preheated oven and bake for 20 to 25 minutes, or until the top turns a glorious golden color and bubbles with delight.
Remove it from the oven and let it cool for a few minutes after baking. Garnish with fresh parsley or chives if desired.
Serve the Outback Steakhouse Mac and Cheese hot as a side dish or as a main course.