Heat your oven to 180°C/Fan 160°C/Gas 4. Prick the potatoes all over with a fork or skewer. Pop on a plate. Microwave on high for 10 mins till they’re tender and cooked through (see our tip if you don’t have a microwave).
Using oven gloves or tongs, transfer the jacket potatoes to a baking tray. Drizzle 1 tbsp oil over the potatoes. Sprinkle with salt and pepper. Bake for 30 mins till the skins are crisp.
Peel and finely chop the onion. Peel and grate or crush the garlic. Warm a deep frying pan or wok over a medium heat. Fry the lardons over a medium heat for 2 mins till they’re starting to brown (you shouldn’t need to add oil as the fat should run out as they cook).
Add the onion to the lardons. Season. Stir and fry for 5 mins till soft. Add the garlic. Stir and fry for 1 min.
Add 1 tbsp mustard, 1 tbsp sugar and the chopped tomatoes to the lardons. Drain and rinse the beans. Add them to the pan. Cover and bring up to a simmer. Cook over a medium heat for 10 mins till thickened. Stir every so often while it cooks.
Pick out any thick stalks from the watercress. Drizzle ½ tbsp oil over the watercress. Season.
Split the jacket potatoes in half. Fluff the potato up with a fork. Top with the baked beans. Serve with the watercress on the side.