French Onion Cabbage Gratin

Name: French Onion Cabbage Gratin
Ingredients: 2 tbsp. unsalted butter; 2 large yellow onions, thinly sliced into half-moons; 1 tsp. kosher salt; 2 thyme sprigs; 1/3 c. dry white wine; Freshly ground black pepper; 1 small head of green cabbage (about 2 lb.), cut into 6 wedges; 1 1/4 tsp. kosher salt; 4 tbsp. unsalted butter, divided; 1/2 c. panko breadcrumbs; 4 tbsp. neutral oil, divided; 2 tbsp. all-purpose flour; 1 1/2 c. heavy cream; 1 1/2 c. whole milk; 1 1/2 c. shredded Gruyère, divided