Put the potatoes in a large saucepan, add cold water to cover by an inch and season generously with salt. Bring to a boil, reduce the heat, and simmer until potatoes are fork tender, about 10-12 minutes depending on their size.
Drain and halve the potatoes if large and set aside while you prepare the broth.
Add the finely chopped garlic along with the tablespoon of olive oil over low heat to a large skillet and cook for a minute until the garlic starts to sizzle and becomes fragrant. Roughly chop the chard and add to the pan along with ½ cup of the stock.
Roughly chop the chard and add to the pan along with ½ cup of the stock. Cook for 2-3 minutes or just until the chard starts to wilt. Add the chard to the bowl of a food processor or blender and pulse until the chard is finely chopped.
Add the potatoes and chard mixture back to the skillet along with the remaining ½ cup of stock and the lemon juice. Cook over medium heat until the potatoes are heated through. Taste and season with salt and pepper.
Cut 2 sheets of foil, about 12-inches long. Divide sausage, potatoes, green beans, mushrooms and onion into 2 equal portions and add to the center of each foil in a single layer.
Fold up all 4 sides of each foil packet. Add butter, cajun seasoning, salt and pepper, to taste; gently toss to combine. Fold the sides of the foil over the sausage, covering completely and sealing the packets closed.
Place foil packets on the grill and cook until just cooked through, about 12-15 minutes.*
Served immediately, garnished with parsley, if desired.
*Foil packets can also be cooked in the oven at 425 degrees for 18-20 minutes.