In a small bowl, mix the Dijon mustard with 1 Tbsp olive oil, minced garlic and lemon juice. Add the pork ribs, coat them thoroughly, cover, and let marinate for at least 2 hours or overnight in the fridge.
Cut the new potatoes into bite-sized pieces. Place them in a pot of cold water, bring to the boil, and cook for 10–15 minutes until just tender. Drain and allow to cool.
Preheat the oven to 170 °C (340 °F).
Arrange the marinated pork in a large roasting tray. Tuck the cooked potatoes and onion wedges between the pieces of meat. Brush the potatoes with the remaining olive oil. Season everything generously with salt and pepper, then sprinkle over the chopped rosemary.
Cover the tray with foil and bake for 30 minutes in the centre of the oven.
Remove the foil, increase the oven temperature to 200 °C (400 °F), turn over the pork, then roast for a further 15 minutes, or until the meat and potatoes are golden.
Serve warm, garnished with extra rosemary if desired.