Caramelize the Onions: Heat 1 tablespoon of olive oil in your 12-inch cast-iron skillet over medium heat. Add the sliced yellow onions and a heavy pinch of salt. Sauté until the natural sugars release and the onions turn a deep, translucent golden brown. Remove and set aside.
Fabricate the Cabbage: Cut the whole green cabbage into 5 to 6 thick wedges. You must leave the core completely intact to maintain structural integrity so the leaves do not shatter in the pan. Season generously with salt.
The Aggressive Sear: In the same hot cast-iron skillet over medium-high heat, place the cabbage wedges flat-side down. Immediately place your secondary heavy pan directly on top to force contact with the metal.
The Maillard Reveal: Sear for 4 minutes until a deep, dark mahogany crust forms. This chemical reaction is mandatory for flavor development. Remove the press, flip the wedges, and turn off the stovetop heat.
The High-Protein Emulsion: In your high-speed blender, combine the ¾ cup of cottage cheese and ⅔ cup of bone broth. Blend on high for 60 seconds until the curds are completely destroyed and a smooth, cream-like liquid forms.
Structural Assembly: Tuck the reserved caramelized onions firmly into the empty crevices between the seared cabbage wedges inside the cast-iron skillet.
The Liquid Pour: Pour the liquid emulsion evenly over the top of the cabbage and onions. It will pool at the bottom to steam and tenderize the tough internal fibers.
The Final Layering: Distribute the 1 cup of grated Swiss cheese and ¼ cup of crumbled blue cheese evenly across the top. Scatter the fresh thyme sprigs and crack fresh black pepper over the dish.
The Golden Crust: Bake in a preheated oven at 400°F (200°C) for 20 to 30 minutes. The gratin is finished when a bubbling, golden-brown shield of hardened cheese forms across the entire surface. Serve immediately.