Preheat your oven to 425°F (220°C) and line a large sheet pan with parchment. This helps the potatoes brown without sticking.

Rinse the baby potatoes and cut them in half. If any are much larger, cut them into quarters so the pieces are similar in size and cook evenly.

Add the potatoes to a pot of salted boiling water and cook for 8–10 minutes. They should be slightly tender on the outside but still firm in the center when pierced with a knife. Drain well and let them sit for a few minutes so steam can escape.

Clean the mushrooms and slice them into thick pieces. Aim for sturdy slices so they keep their shape and caramelize instead of shrinking into thin, watery pieces.

In a large bowl, whisk together the olive oil, Dijon mustard, and herbes de Provence. The mixture should look smooth and lightly thickened from the mustard so it clings to the vegetables.

Add the parboiled potatoes and sliced mushrooms to the bowl. Toss until every surface looks lightly coated with the herb mixture. Sprinkle with salt and pepper to taste.

Transfer the vegetables to the prepared pan and spread them into a single layer. Leave small gaps between pieces so the heat can circulate and encourage browning.

Roast for about 30 minutes, stirring once halfway through. The potatoes are ready when they are deeply golden at the edges and the mushrooms look browned and slightly crisp in spots. Serve hot.