Pat the sardines completely dry with paper towels. Season both sides lightly with salt and pepper.
Heat 2 tablespoons olive oil in a wide, heavy skillet over medium-high heat until it shimmers. Lay the sardines skin-side down and press gently for the first 20 seconds. Cook undisturbed 2 to 3 minutes until the skin releases, then flip and cook 1 more minute. Remove to a plate.
Lower the heat to medium. Add the remaining olive oil, harissa paste, garlic, cumin, and smoked paprika. Stir constantly 30 to 40 seconds until fragrant.
Pour in the stock, scraping up browned bits. Bring to a gentle simmer.
Add the bok choy stems first, cover, and cook 90 seconds. Add the leafy tops, cover, and cook 60 to 90 seconds more until wilted but bright green.
Nestle the sardines back into the pan on top of the bok choy. Spoon harissa broth over the fish and warm through 1 minute.
Finish with lemon juice, scatter parsley or cilantro on top, and serve straight from the pan with lemon wedges.