At least 7 hours and up to 1 day before you plan to bake, prepare the dough: Preheat the oven to 400°F. Puncture the potato with a fork multiple times, wrap it in foil, and bake until it is completely tender when poked with a paring knife, 45–60 minutes. Remove and let cool slightly, then peel and discard the skin.
Place the potato in a medium bowl; use a fork to coarsely mash, then set aside at room temperature to cool completely.In medium bowl, whisk ⅔ cup cool water, the yeast, honey, and ½ cup of the flour until completely combined; cover the bowl with plastic wrap and set aside until the mixture is very bubbly, 50–60 minutes.
Once this happens, finish the dough: Unwrap the bowl and add the mashed potato, 1 cup of tepid water, the olive oil, kosher salt, 2½ cups all-purpose flour, the semolina, and the 1 cup of finely chopped kale. Stir with a fork until a dough begins to form.
Lightly flour a work surface and turn the dough out onto it. Press and knead to form a ball. Knead the dough, adding a bit more flour as needed, until it is homogenous and smooth but still soft and slightly sticky, 3–5 minutes. Lightly oil a large bowl and place the dough in it. Cover the bowl with plastic wrap and set aside until the dough feels slightly puffed when squeezed, 20–30 minutes.
Generously oil a large rimmed baking sheet. Transfer the dough to the baking sheet, stretching and pressing it lightly to flatten (it will not completely fill the pan at this stage). Wrap the baking sheet tightly with plastic wrap and refrigerate for at least 6 hours or overnight.
Two hours before you plan to cook the focaccia, preheat the oven to 475°F. Retrieve the baking sheet and allow the dough to come up to room temperature and puff up, about 1¾ hour. Unwrap, then use your fingers to lift and poke the dough so that it fills the corners of the pan and is in a generally even layer (try not to excessively deflate it as you work). Top the dough with the remaining cup of torn kale and drizzle with more olive oil, then use your fingers to dimple the dough a bit more, pressing the greens into the surface. Sprinkle with the red pepper flakes to taste, and the sea salt.
Bake until the kale is crisp at the edges and the bread is evenly golden brown, 20–25 minutes. (Take care when retrieving the hot pan from the oven as there may be pockets of hot olive oil remaining around the bread.) Let cool slightly. If desired, transfer to a cooling rack to cool completely. Cut into strips and serve within a few hours.