Spray an 8-inch microwave-safe baking dish with nonstick cooking spray. Place 1/3 of the potatoes and 1/2 of the onions on the bottom of the dish and sprinkle with 1/3 the cheese and 1/2 the herbs.

Repeat layers, then top with the last 1/3 of the potatoes, layering potatoes so that there is a solid layer of potatoes with no gaps; sprinkle with remaining cheese.

Stir together stock, Dijon mustard, and garlic salt, and pour over the potatoes.

Cover with plastic wrap and microwave on HIGH until internal temperature registers 165 °F on a food thermometer, about 20 minutes.

Use oven mitts or a towel to remove dish from microwave; carefully remove cover due to steam build-up and serve.

Use regular salt and a dash of garlic powder in place of garlic salt if you like.