Add butter and harissa to a small bowl, stirring to combine. Season to taste with kosher salt and a squeeze of lemon juice. This makes enough Harissa Butter for 2 slices of toast. Set aside.
Preheat an oven to 450°F. Toast bread in the oven until lightly golden (about 5 minutes). Spread half of the Harissa Butter on the toast, then top with sliced hard-boiled egg and several small parsley leaves. Season to taste with kosher salt, freshly ground black pepper and a squeeze of lemon juice. Store any unused Harissa Butter in the refrigerator for up to 2 weeks.