Bring a large pot of lightly salted water to a boil. Add the parsnips and boil until firm-tender, 3 to 5 minutes. Drain and spread on a rimmed baking sheet to cool to room temperature.
Heat 2 Tbs. of the olive oil in a large cast-iron skillet over medium-high heat. Add the parsnips, a pinch of salt, and a few grinds of pepper and cook, stirring occasionally, until softened and lightly browned, about 4 minutes. Add the pancetta and cook until sizzling and crisp, about 2 minutes. Add the sage and garlic and cook, stirring frequently, until the garlic is fragrant and the sage is starting to crisp, about 2 minutes more. Set aside.
Heat the remaining 2 Tbs. olive oil in an 11- to 12-inch straight-sided sauté pan over medium-high heat. Add the onion, a small pinch of salt, and a few grinds of pepper and cook, stirring occasionally, until beginning to soften, about 5 minutes.
Add the rice, 2 tsp. salt, and the saffron (if using), stirring well to coat. Add 2 cups of the broth and the wine; simmer, stirring, until the liquid is completely absorbed, 3 to 4 minutes. Continue adding the broth in 1 cup increments, stirring and adjusting the heat to maintain a brisk simmer and letting each addition be almost absorbed before adding the next. The risotto is done when the rice is nearly but not fully tender (al dente) and still a little soupy (this usually takes 14 to 16 minutes after the first addition of liquid). You may not use all the broth, but you should use at least 4 cups.
Fold the parsnip mixture into the risotto. Add the butter and Parmigiano and stir gently to incorporate. Season to taste with salt and pepper. Serve sprinkled with more grated Parmigiano.