This hearty, wintry dish is a cross between a shepherd's pie and potato gratin. It's got a layer of browned ground beef spiked with onions, sage and spinach on the bottom, with a luscious, cheese- and cream-slathered root vegetable topping that turns golden and crisp-edged in the oven. If you aren't a rutabaga fan, you can use all potatoes, or a combination of white and sweet potatoes. This gratin reheats very well, so feel free to make it ahead and reheat it uncovered in a 350-degree oven. And although it qualifies as a one-pan meal (with meat, green vegetable and starch altogether), a fresh and tangy green salad on the side would round things out nicely.
03Add onion to pan drippings (drizzle with oil if pan seems dry). Cook, stirring occasionally, until onions are tender and golden, about 10 minutes. Peel and chop remaining 2 garlic cloves; add to pan with remaining sage. Return meat to skillet. Toss in spinach, a handful at a time, until wilted. Season with 1/4 teaspoon salt, the Worcestershire and pepper to taste.
04Heat oven to 350 degrees. Peel rutabaga and cut in half. Slice each half crosswise into 1/8-inch-thick slices. Peel potatoes and cut into 1/4-inch-thick rounds. Layer half the rutabaga and potato slices over meat, alternating between rutabaga and potato, with slices overlapping one another. Season lightly with salt and pepper; top with half the cheese. Repeat with remaining vegetables and cheese. Spoon reduced cream evenly over top.
05Cover pan tightly with foil and bake until vegetables are very tender, 60 to 75 minutes. Uncover and cook until golden brown, about 10 minutes more. Cool 10 minutes before serving.