Preheat the oven to 375°F and lightly grease a baking dish.
Bring a large pot of water to a boil and salt it generously.
Add the macaroni and cook until just tender, then drain without rinsing.
In the same pot, melt the butter over medium heat.
Whisk in the flour and cook for about one minute until smooth and pale.
Slowly whisk in the milk, keeping the heat steady and stirring constantly.
Cook until the sauce thickens and coats the back of a spoon.
Season the sauce with salt, black pepper, mustard powder, and smoked paprika.
Reduce the heat to low and add the mild cheese in handfuls, stirring until fully melted.
Stir in the sharp cheese and continue stirring until the sauce is smooth.
Add the lemon juice and gently stir to brighten the flavor.
Fold the cooked macaroni into the cheese sauce until evenly coated.
Transfer the mixture to the prepared baking dish and smooth the top.
Melt one tablespoon butter in a small pan over medium heat.
Add the breadcrumbs and toast until golden and crisp.
Sprinkle the toasted breadcrumbs evenly over the mac and cheese.
Bake uncovered for 20 to 25 minutes until bubbling at the edges and lightly golden on top.
Remove from the oven and let rest for 5 minutes before serving.