Heat 2 tbsp oil in 12 inch non stick skillet over medium heat until shimmering. Add potatoes, mushrooms, onions, 1/2 cup water and 1/4 teaspoon salt. Cover and cook, stirring occasionally until vegetables are softened, 8 to 10 minutes. Uncover and continue to cook, stirring occasionally, until evaporated and potatoes are golden brown. Stir in the garlic and cook until fragrant. Transfer to a bowl and cover to keep warm.
Beat eggs, 3/4 teaspoon salt, 1/4 teaspoon pepper, and remaining 2 tablespoons water together with fork in a bowl until thoroughly combined and mixture is pure yellow.
Wipe skillet clean with paper towels and heat remaining 2 tablespoons oil. Add the egg mixture, constantly scraping the bottom and sides of the skillet until eggs begin to clump, 1 1/2 to 2 minutes.
Reduce heat to low and gently and constantly fold eggs until clumped and slightly wet. Off the heat gently fold in potato mixture. Drizzle with the harissa and sprinkle with cilantro. Serve immediately, passing extra harissa.