Combine potatoes and onion in a microwave-safe bowl. Sprinkle garlic powder over mixture and add water. Cover bowl and heat in microwave on high until potatoes are tender, 8 to 10 minutes.
Combine butter and flour in a saucepan over medium heat; cook, stirring constantly, until butter is melted, 2 to 3 minutes. Slowly add milk and cook, stirring constantly, until mixture is the consistency of gravy, 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 potatoes; stir to coat. Cover bowl and microwave on medium until flavors combine, 3 to 5 minutes. Let stand for 5 minutes before serving.