Bring first 3 ingredients to boil in heavy large skillet over medium heat. Cover and simmer until lemons are tender, about 12 minutes. Uncover; simmer until liquid thickens to syrup, about 8 minutes. Transfer syrup to bowl and cool. Chop lemon slices; add to bowl and cool. Chop lemon slices; add to bowl. Mix in herbs, oil, and garlic.
Heat oil in heavy large skillet over medium heat. Add onions; sprinkle with salt and pepper. Cover; cook until onions are golden, stirring often, about 25 minutes. Add 1/2 cup water and 1 tablespooon harissa (or sambal). Simmer until water is absorbed, 4 to 5 minutes. Add more harissa, if desired.
Spread hummus on buns or crosswise in strip on each hot dog bun. Top with eah with grilled hot dog, then onions and relish. Roll up hot dog bun and serve.