Heat 2 tablespoons of oil in a large skillet. Cook pancetta until crispy, about 6 minutes. Add in leeks and garlic and cook until soft and starting to brown, about 10 minutes.
Add the wine and reduce by about 1/3, about 5 minutes. Add in plums, cover, and cook for about 5 more minutes. Stir it once or twice in between.
Then place mixture in large bowl and let cool to room temperature. Stir in the bread cubes, thyme, and 1/4 cup of parsley. Season with salt and pepper. This will yield way more stuffing than is required for the 4 birds. It was pretty tasty so you could even cook it in a separate pan alongside the birds if you wanted extra stuffing.
Fill the cavities of the hens with the stuffing. Tie the legs together with some kitchen string. Spread the parsnips in an even layer on a large rimmed baking sheet. Use the remaining 1/4 cup of oil to coat the parsnips. Season with salt and pepper. Place hens, breast side up, on top of parsnips, drizzle with a little olive oil and season with salt and pepper. Then turn hens over, breast side down.
Pour chicken stock into baking sheet. Cover everything with foil and roast in oven for 45 minutes. Be careful, this is kinda heavy, and if you wanted to, you could even split it into 2 baking sheets, but I managed to put all 4 hens and everything in 1.
After the 45 minutes, remove foil, turn over hens (breast side up) and roast for another 30 minutes. The hens will be a nice golden brown to let you know they're done.
Let the hens rest on a work surface for 10 minutes.
Using a slotted spoon, scoop up the parsnips and place them in a platter. Garnish with remaining 1/4 cup of parsley.
Transfer the pan juices from the baking sheet into a small bowl. Skim off any excess fat and season with salt and pepper.
Transfer hens to a plate and serve with parsnips, stuffing, and pan juices. I also made an acorn squash puree that went well with the birds also (roast 2 halves of an acorn squash for 1 hour, scoop out filling, season with parmesan cheese, butter, salt and pepper, and puree with immersion blender, voila... acorn squash puree).