If you come across turnips with good-looking greens still attached, use in place of the kale.
- 5 garlic cloves, thinly sliced
- 2 tablespoons unsalted butter, divided
- 3 medium onions, thinly sliced
- 3 bunches Tuscan kale, ribs and stems removed, leaves torn
- 4 medium turnips (about 1¾ pounds total), trimmed, peeled, cut into ½-inch pieces
- 3 large eggs, beaten to blend
- 4 ounces Fontina cheese, grated (about 1 cup)
- 1 ounce Parmesan, finely grated (about 1 cup)
- 8 ounces day-old white country-style bread, cut into ½-inch pieces
- Freshly ground black pepper
Bring garlic, cream, and thyme to a bare simmer over medium heat. Reduce heat to low and let cream simmer 30 minutes. Let cool.
Meanwhile, heat 1 Tbsp. butter in a large skillet over medium-low. Add onions, season with salt, and cook, stirring occasionally and adding a splash or two of water if onions begin to stick to pan, until caramelized and amber colored, 45–60 minutes. Transfer to a large bowl and let cool. Wipe out skillet.
Heat remaining 1 Tbsp. butter in same skillet. Working in batches, add kale, tossing and letting it wilt slightly before adding more; season with salt. Cook until kale is wilted and tender, 5–8 minutes; transfer to bowl with onions.
While kale is cooking, cook turnips in a large pot of boiling well-salted water until crisp-tender, about 2 minutes; drain. Transfer to a bowl of ice water and let cool. Drain; pat dry. Transfer to bowl with onions.
Preheat oven to 375°. Whisk eggs, Fontina cheese, Parmesan, and cooled cream mixture in a large bowl to combine. Add onion mixture and bread; season with salt and pepper. Transfer to a 13x9" baking dish and press down on mixture with your hands to form a tight, even layer. Bake gratin, uncovered, until well browned, 40–50 minutes. Let rest 10 minutes before serving.
Do Ahead: Gratin can be assembled 12 hours ahead. Cover and chill.