1. Heat olive oil in a large skillet over medium heat. Add garlic and cook, stirring occasionally until the slivers are nicely golden (but not too dark), 2–3 minutes. Add lemon slices and season with salt and pepper. Cook, stirring occasionally until they’ve also started to turn a nice golden brown and frizzle at the edges, 3–4 minutes.
2. Add harissa paste and cook a minute or two in the oil, frying the spices and caramelizing the paste to take the edge off. Add shrimp and a few tablespoons of butter (I think more is better here) and season with salt and pepper. Cook, tossing until the shrimp are nicely coated, have turned a deep, bright pink, and are noticeably firmer, 4-6 minutes.
3. Remove from heat, squeeze the other half of lemon over the shrimp, and toss to combine before transferring it to a platter. Serve it with baguette to sop up the sauce.
*Harissa paste, a North African condiment made with chili and spices like cumin and coriander, can be radically different depending on the brand, the ingredients and where it comes from. Some are almost pure chili (lots of heat), some extremely mild, containing ingredients like sundried tomatoes or roasted red peppers for body and sweetness. My preference is to always have two types on hand for different applications, but if I had to choose one, it would be one with more heat than sweetness.