A quick note on cooking methods: a jacket potato is never made in a microwave. A baked potato, sure, if you're short on time, but it'll never develop the crisp skin and fluffy inside that defines the jacket potato.
1 largish potato (floury variety)
Rock salt (i.e. coarse grains)
Half a courgette
2–4 cloves of garlic (depends on how garlicky you want the dish to be)
1 tablespoon of capers
2 tablespoons tomato purée
1 tsp dried / 1 tbsp fresh Basil
1/2 tsp dried / 1/2 tbsp fresh Oregano
Salt & Pepper, to taste
Grated Cheese (optional)
Begin by heating the oven to about 180℃. Clean and scrub the potato, and coat liberally in oil - olive oil may be healthier but doesn't quite crisp up in the way sunflower oil does. Then rub the sock salt into the oily potato. This draws out the moisture from the skin of the potato, again helping to create the desired crispy jacket. Whack the potato into the oven for an hour to an hour and a half, turning halfway through. I've yet to work out whether covering the potato in tin foil is at all beneficial, but intuition says it would keep the moisture in, delaying the development of a truly crisp skin.
For the filling, thinly chop the courgette and garlic, and fry in a mixture of butter and olive oil for about 5 minutes, until the mixture begins to brown slightly. Roughly tear up the herbs (if fresh) and add to the mixture, along with the capers, salt and pepper. Stir in the tomato purée and cook off for about 5 minutes more to let the flavours develop. Simple!
The potato is only done when the skin has browned and crisped up - the inside may be cooked long before that point, but remember this is a jacket potato, not merely a baked potato. The skin should audibly crack when you cut it open, and stick into your gums when you try and eat it.
When it's done, slice open, ladle on the topping and cover in more more salt and pepper if needed. I added cheese because I just needed it. Not a posh cheese either - it has to be the cheap kind that melts easily and coasts everything in a lovely fatty layer.
The filling may seem weird in that it doesn't feature onions (an omission caused by not having any in the house) but it didn't feel at at lacking. The beauty of the filling is that it is infinitely variable. Some sun-dried tomatoes, a couple of anchovies, or roasted aubergine would all have gone well. It may take a long time start to finish, but the effort level is so ridiculously low it hardly matters.