You can put anything in a stew but here's a combination that turned out nicely. It's cooked slowly (you ignore it for about 2 hours), which means that the starch from the potatoes thickens the broth up nicely, and also that the flavours from the leek/apricot/etc all melt together nicely. That's why the stew doesn't really need any flour or stock.
Warm up a couple of tablespoons of veg oil in a large deep pan on a medium heat. Remove the green bit from the leek and save it (added later), then finely slice the white bit into lots of rings. Add the rings to the pan and stir them round. Let this fry a little bit while you peel & slice the garlic and add it. Stir around and let fry for a couple of minutes until starting to soften.
Add the diced beef to the pan and stir again. Let it fry for a minute or so while occasionally stirring, then leave it to cook a bit more while you do the potatoes. Peel the potatoes and chop them into bitesize chunks about the same size as the pieces of beef. Add them to the pan. Chop the apricots in half and add them to the pan too.
Then add enough boiling water to cover everything, and turn the heat up. You can add the marrow now or you can wait for an hour or so if you'd prefer to avoid it going too mushy. Either way, you just chop it into large pieces (a bit bigger than the potato pieces) and add it to the stew.
When the stew is bubbling nicely, put a lid on and turn the heat right down as far as it'll go. After a couple of minutes you can tell by feel/sound that there's still some bubbling going on in there despite the low heat. Leave this to bubble on its own for two hours.
Optionally, half an hour before serving, take the green bits of the leek, get rid of any manky bits, and slice it all up, then add it to the pan and give it all a stir.