Preheat oven to 425 degrees. Cut baby potatoes in half and/or quarters depending on size, so they are all uniform in shape.

Cut onions in half, then slice into 1/2 inch strips.

In a large bowl, toss the potatoes and onions with the olive oil, mustard, thyme leaves, and salt and pepper.

Line a rimmed baking sheet with parchment paper. Pour potato and onion mixture onto the baking sheet and spread it out evenly.

Cook the potatoes in the oven for 1 hour, toss them with a spatula every 15 minutes to ensure even roasting.

Remove from oven and serve hot.