In a microwave-safe bowl, combine 6 sliced potatoes and 1 large cut onion.
Sprinkle 1/4 teaspoon garlic powder over the mixture and add 1/4 cup water.
Cover the bowl and microwave on high until potatoes are tender, about 8 to 10 minutes.
In a saucepan over medium heat, combine 3 tablespoons butter and 2 tablespoons all-purpose flour.
Cook, stirring constantly, until butter is melted, about 2 to 3 minutes.
Slowly add 1/2 cup milk and cook, stirring constantly, until the mixture reaches the consistency of gravy, about 2 to 3 minutes.
Reduce heat, stir in Dijon-mayonnaise blend, and mix well.
Remove from heat and cover.
Drain water from potatoes and pour gravy mixture over them, stirring to coat.
Cover the bowl and microwave on medium until flavors combine, about 3 to 5 minutes.
Let the dish stand for 5 minutes before serving.