Place the couscous in a bowl, add a pinch of sea salt and black pepper, then just cover with boiling kettle water, and cover. Peel and slice the garlic, then place in a shallow casserole pan on a medium-high heat with 1 tablespoon of olive oil, stirring while you halve the cherry tomatoes, adding them to the pan as you go. Snap the woody ends off the asparagus, roughly chop the spears and stir into the pan, then season with salt and pepper. Rub most of the harissa over the fish, sit it on top of the veg, finely grate over the lemon zest and squeeze over half the juice. Add 150ml of water, cover, and cook for 5 minutes, or until the fish is just cooked through.
Fluff up the couscous. Ripple the remaining harissa through the yoghurt, then spoon it over the fish and couscous. Serve with lemon wedges, for squeezing over.