Cut halloumi into ½" (1 cm) slices. Heat a large oven-safe skillet over medium heat, add the olive oil and heat through.
Add the slices of halloumi and fry until golden brown, about 1 minute. Flip and continue to cook until golden brown, about an additional minute. Remove with tongs to a plate.
Preheat the oven to 375°F (190ºC). Return the pan with the oil to medium heat. Add the onion and cook, stirring occasionally, until soft and translucent, about 6 minutes.
Add garlic and spices
Add the garlic and cook until fragrant, about 30 seconds. Stir in the cumin and paprika, and cook for 1 additional minute.
Make harissa tomato sauce
Add the tomatoes and harissa, then season with salt and pepper. Simmer until the tomato sauce has thickened slightly, about 15 minutes.
Add eggs and cook
Gently make little holes in the sauce and crack the eggs into the skillet over the tomatoes. Transfer the skillet to the oven, and bake until the eggs are just set, 8 to 10 minutes.
Assemble and serve
Break the fried halloumi into pieces and scatter over the shakshuka. Top with cilantro and green onions. Serve immediately with grilled challah bread on the side.