Preheat the Oven: Position a rack in the center of the oven and preheat to 425°F (220°C). Line a large baking sheet with parchment paper if desired. Preheating the oven ensures that the potatoes roast evenly and develop a nice golden color.

Season the Potatoes: Place the baby potatoes in a large bowl. Toss with the olive oil, salt, black pepper, and garlic powder until evenly coated. This step is crucial as it helps the seasonings adhere to the potatoes and enhances their flavor during roasting.

Roast the Potatoes: Arrange the potatoes cut-side down in a single layer. Roast for 35 to 40 minutes, turning once halfway through, until the potatoes are golden brown and easily pierced with a fork. If they are not yet tender, continue roasting in 5-minute intervals. The goal is to achieve a crispy exterior while keeping the inside fluffy.

Cook the Garlic: About 10 minutes before the potatoes finish roasting, melt the butter in a large skillet over medium-low heat. Add the minced garlic and cook for about 1 minute, stirring constantly, until fragrant. Do not allow the garlic to brown, as this can lead to a bitter taste in the sauce.

Build the Sauce: Sprinkle the flour over the garlic butter and whisk continuously for 1 minute. Slowly whisk in the milk and heavy cream until smooth. This step creates a roux that will help thicken the sauce, ensuring a creamy texture.

Thicken the Sauce: Bring the mixture to a gentle simmer over medium-low heat, whisking frequently for 3 to 5 minutes until it lightly coats the back of a spoon. This simmering process allows the flavors to meld and the sauce to thicken properly.

Finish the Sauce: Reduce the heat to low: Stir in the Parmesan cheese, Dijon mustard, dried parsley, salt, and black pepper until smooth and creamy. If the sauce becomes too thick, whisk in 1 to 2 tablespoons of milk. The cheese adds richness and depth to the sauce, making it more flavorful.

Coat the Potatoes: Add the hot roasted potatoes to the skillet and gently fold until every potato is coated with the garlic sauce. Avoid stirring too vigorously to prevent breaking the potatoes. This ensures that each potato is evenly covered in the creamy sauce without losing its shape.

Serve: Transfer the potatoes to a serving dish: Sprinkle with fresh parsley and additional Parmesan cheese, if using. Serve immediately while hot. Serving right away keeps the potatoes warm and the sauce creamy.