Bring a large pot of generously salted water to a full boil. Add the chopped kale and blanch for exactly 2 minutes. The color will turn a bright, vivid green. Drain immediately into a colander and rinse with cold water to stop the cooking.

Working in small handfuls, squeeze the blanched kale firmly over the sink to remove as much water as possible. This step matters. Wet kale will water down your sauce and prevent it from coating the greens properly. Squeeze until each handful feels almost dry, then set aside.

Melt the butter in a large skillet over medium heat. Add the sliced garlic and cook, stirring frequently, for 3 to 4 minutes. You want the garlic to turn pale golden and smell nutty, not brown or bitter. Watch it closely because sliced garlic moves from toasted to burnt quickly.

Sprinkle the flour over the toasted garlic and stir to coat everything. Cook for 1 minute, stirring constantly, to cook out the raw flour taste. Pour in the heavy cream and milk in a slow, steady stream, whisking as you go to break up any lumps. Raise the heat slightly and stir until the sauce thickens enough to coat the back of a spoon, about 4 to 5 minutes.

Reduce the heat to low and add the shredded white cheddar a handful at a time, stirring after each addition until fully melted and smooth. Season with black pepper, nutmeg if using, and taste for salt.

Add the squeezed kale to the skillet and fold it into the sauce with a wooden spoon until every piece is well coated. Cook for 2 to 3 minutes more over low heat, stirring occasionally, until the kale is heated through and the sauce has settled into a thick, glossy coating. Serve hot.