Give the sweet potato a good scrub, pat dry, and cut into ¼” cubes. Heat a cast-iron skillet over medium heat and add the olive oil followed by the sweet potatoes. Cook the sweet potatoes, stirring every couple of minutes, until the sweet potatoes are tender and starting to brown, 10 to 15 minutes (depending on how hot your skillet is).

Stir in the garlic and salt; cook for about a minute more, until the garlic is fragrant. Turn off the heat and stir in the harissa paste. 

While the sweet potatoes are cooking, place the parsley on a cutting board and add the lemon zest on top. Mince until combined and the herbs are extremely fine. 

Toast the bread and divide the ricotta on top of each piece of bread. Divide the sweet potatoes on top of the ricotta and finish with a heavy sprinkle of the herb mixture.