Fontina is fine here, but if you can splurge on truffled cheese, you won't regret it.
About 1 1/2 tbsp. kosher salt
3 bay leaves
2 cups coarse polenta (not instant)
About 4 tbsp. unsalted butter, divided
1 1/2 large yellow onions, minced (2 1/2 cups)
3/4 pound truffled cheese, such as Sottocenere al Tartufo or Caciotta al Tartufo, or Italian-style fontina, coarsely grated
1/3 cup finely shredded parmesan cheese (from a wedge)
How to Make It
Bring 7 1/2 cups water to a boil in a large pot over high heat. Add 1 1/2 tbsp. salt and the bay leaves. Whisking constantly, gradually add polenta. Reduce heat to medium and cook, whisking often, until mixture begins to thicken, then adjust heat to maintain a gentle bubble. Cook, stirring occasionally with a sturdy rubber spatula and scraping pot, until polenta is thick and creamy and grains are softened, 30 to 45 minutes. If it becomes too stiff to stir easily, add a little boiling water.
Melt 3 tbsp. butter in a medium nonstick frying pan over medium-low heat. Add onions and cook, stirring often, until soft and pale golden, about 20 minutes.
Preheat oven to 375°. Remove bay leaves from polenta. Stir in onions and season with salt to taste.
Butter a 13- by 9- by 2-in. baking dish generously. Pour one-third of polenta into dish, spreading evenly. Top with half of truffled cheese. Add another one-third of polenta, then remaining truffled cheese. Dollop remaining polenta on top and spread evenly. Sprinkle with parmesan and dot with remaining 1 tbsp. butter.
Bake torta until medium brown, bubbling on the edges, and hot to the touch, 40 to 45 minutes. Let cool, then chill until cold, at least 2 hours and up to 1 day.
Cut torta into small rectangles (about 1 1/2 by 2 in.) and transfer to a baking sheet. Broil 4 inches from heat until browned and crisp, about 5 minutes.
Make ahead: Through step Cover with plastic wrap and chill several hours or overnight.