Cover the rutabaga and potatoes with cold, generously salted water in a large pot and bring to a boil over medium-high heat. Reduce the heat and simmer until the vegetables are fork-tender, about 30 minutes.
Drain the vegetables and return to the pot over low heat. Stir in 8 tablespoons butter. Mash with a potato masher, ricer or food mill until smooth, adding the warm half-and-half and 2 teaspoons salt. Keep warm while preparing breadcrumbs.
Melt the remaining 4 tablespoons butter with the olive oil in a large skillet over medium-high heat. Add the lemon zest and scallions. Add 1/2 teaspoon salt and cook, stirring, until fragrant, about 2 minutes. Add the parsley and breadcrumbs and cook over medium-low heat, stirring until evenly toasted, 3 to 4 more minutes. (If the breadcrumbs toast too quickly, reduce the heat.)
Transfer the mashed vegetables to a large shallow platter and sprinkle with the breadcrumbs just before serving. Garnish with scallion slices. (To curl them, julienne whole scallions and place in ice water.)
Photograph by Anita Calero
Courtesy of Food Network Magazine