In a deep-sided skillet or a Dutch oven over medium-high heat, heat oil until shimmering. Add onion, season with salt, and stir occasionally until they start to soften and become translucent in color, about 4 minutes, lowering heat if they start to brown. Add garlic and harissa, stirring until fragrant, about 30 seconds.
Carefully pour in tomatoes and their juices, and water — watch out for splattering! Bring to a boil, reduce to a simmer, and season with salt and pepper.
Add clams, cover, and adjust heat to maintain an active simmer. Cook until most of the clams open, 7 to 9 minutes.
Ladle into serving bowls, discarding any unopened clams, and sprinkle with parsley leaves. Serve with bread.