Grate the cheeses and bring cream cheese to room temperature. Freshly shredding cheddar and Gruyère from blocks (rather than using pre-shredded) is crucial because pre-shredded cheeses contain anti-caking agents that prevent smooth melting. Set the cheeses aside in two bowls (one for the cheddar + Gruyère, one for Parmesan). Leave the cream cheese out for 10–15 minutes so it blends smoothly into the sauce.

Lightly butter the inside of a 5–6 quart slow cooker or spray with nonstick spray. Rubbing a little butter around the crock prevents sticking and helps the pasta slide as it cooks, reducing the chance of clumping.

In a small skillet over medium heat, melt the 4 tablespoons of butter and add the Dijon, smoked paprika, garlic powder, and onion powder. Cook for 30–45 seconds, stirring constantly, to bloom the spices and cook off the raw mustard edge — this develops deeper, more rounded flavors than adding the dry spices straight to the crockpot.

Pour the butter-spice mixture into the slow cooker. Add the 4 cups whole milk and 1.5 cups water and whisk gently to combine. Use room-temperature milk if possible to avoid dramatic temperature shock to the cream cheese later; the extra water ensures the pasta has room to expand and prevents the milk from boiling over.

Stir in the kosher salt and half of the black pepper. Taste a spoonful of the milk (it will be warm) and adjust seasoning lightly — the cheese will add salt later, so err on the side of light salting now.

Add the dry elbow macaroni and stir to evenly distribute. Push the pasta down so it's submerged but do not press it hard (it needs space to swell). Distributing noodles evenly prevents dry pockets and encourages uniform cooking.

Cover and cook on LOW for 1 hour 45 minutes to 2 hours. Slow-cooker temperatures vary; cook time aims to hydrate and cook the dry pasta gently in the milk so the starches release gradually and create a silky body. Avoid cooking on HIGH for long periods — milk can curdle and pasta can go gummy.

At about 60–75 minutes (when pasta has absorbed a lot of liquid but is still firmer than you want), remove the lid and stir thoroughly to prevent sticking. This midpoint stir helps redistribute heat and liquid and prevents a starchy layer from clinging to the bottom.

When the pasta is just about al dente (about 5–10 minutes before desired doneness; test a piece), add the room-temperature cream cheese in dollops, the shredded cheddar + Gruyère, and the Parmesan. Stir gently but thoroughly until all the cheeses melt into the sauce. Adding cheese late protects texture and prevents oils from separating; low-and-slow cheese melt with cream cheese creates a stable, creamy emulsion.

If the sauce seems thin after the cheese melts, whisk together 1 tablespoon cornstarch with 2 tablespoons cold water to make a slurry and stir it into the crockpot; then cook another 5–10 minutes with the lid on to thicken. Cornstarch needs heat to activate; adding it at the end gives you control over final viscosity without overcooking pasta.

Taste and adjust seasoning with remaining black pepper and more salt if needed. Stir in a final crack of black pepper. For brightness, you can stir in 1 teaspoon lemon juice or 1 tablespoon chopped fresh parsley at this point to cut the richness (optional).

For a golden crust (restaurant flourish): transfer the mac and cheese to a buttered 9x13-inch baking dish, sprinkle a thin layer of extra shredded cheddar and 2 tablespoons panko breadcrumbs brushed with 1 teaspoon olive oil, and place under a hot broiler for 2–4 minutes until bubbling and golden. Watch closely — broilers go fast. The broiler step adds contrast in texture (a crisp top over a creamy base) reminiscent of professional plating.

Serve immediately garnished with chopped parsley. The sauce will thicken slightly as it cools — if you need to reheat, stir in 2–3 tablespoons milk and gently warm on low so you don’t break the emulsion.