Crush and peel 2 garlic cloves.  In a small pan on medium heat, combine the cream, crushed garlic, thyme, and ½ Tbs. sage.  Bring to a simmer (watch carefully as it will boil fairly quickly), reduce the heat and simmer until reduced by half (about 30 minutes.)  Strain and cool.

While the cream is cooling, heat the oil in an ovenproof 10-inch skillet (preferably cast iron) on medium-high heat. Add half the beef and brown it well, crumbling with a fork as it cooks.  Season with ¼ tsp. salt and a few grinds of pepper and transfer to a paper towel-lined plate.  Repeat with the remaining beef, ¼ tsp. salt and the pepper.

In the now-empty skillet, add the onion to the pan drippings (add a little oil if it looks too dry) and cook, stirring occasionally until the onions are tender and golden brown (about 10 minutes.)  Peel and mince the remaining 2 garlic cloves and add to the pan with the remaining sage until the garlic is fragrant, about 30 seconds.  Return the meat to the skillet and add the spinach, a handful at a time, until it is wilted.  (You may have to cover the pan to get the spinach to wilt.)  Season with another ¼ tsp. of salt, pepper to taste, and the Worcestershire sauce.

Heat the oven to 350° F.  Peel the potatoes and cut into ¼-inch rounds.  Layer half of the potato slices over the meat mixture, overlapping the slices slightly.  Season lightly with salt and pepper and add half the cheese.  Repeat with the rest of the potato slices and the cheese.  Spoon the reduced cream evenly over the top.

Cover the skillet tightly with aluminum foil and bake until the potatoes are very tender, about 60-75 minutes. Remove the foil from the pan and cook for another 10 minutes, until the top is brown.  Let sit for 10 minutes before serving.