Heat oven to 400 F with racks in the middle and lowest position. Have rimmed baking sheets in the oven as it preheats.
In a large bowl, toss the potatoes, garlic and shallots with the oil, salt and pepper.
Remove the baking sheets from the oven and add potato shallot mixture. Spread an even layer, ensuring potatoes don't touch and have plenty of room to roast.
Roast on the bottom rack until the potatoes are well browned on the bottoms, about 20 minutes, rotating the sheet halfway through.
Use a thin metal spatula to transfer the potatoes to the bowl. Don't forget to scrape up the browned bits, they're delicious!
Add 4 tablespoons of harissa paste and toss until evenly coated. Return the potatoes to the sheet, spreading an even layer and reserving the bowl. Roast on the middle rack until tender, 18-20 minutes, rotating the sheet half way.
Return the potatoes to the bowl, scraping up any browned bits. Add the parsley, remaining 2 tablespoons of harissa and lemon juice. Toss to coast. Serve with lemon wedges.