In a large pot, heat the EVOO over medium. Add the onion and cook, stirring often, until softened, about 3 minutes. Add the garlic and cook, stirring, until fragrant, 1 minute. Transfer 1/3 cup of the onion mixture to a large bowl.
Add the mushrooms to the pot and cook, stirring occasionally, until just tender, about 3 minutes. Add the stock and couscous to the pot; bring to a simmer and season with salt and pepper.
Add the ground lamb, half the dill, 1 tsp. salt and 1/2 tsp. pepper to the bowl with the onion mixture; mix well to combine. Form into 1-inch meatballs and transfer to the soup pot. Simmer until the meatballs are cooked through and the couscous is tender, about 15 minutes. Remove from the heat and stir in the spinach and remaining dill; season.