Charred Cabbage Skillet Gratin

Name: Charred Cabbage Skillet Gratin
Ingredients: 1 (2-pound) Savoy or green cabbage; 3 tablespoons olive oil, divided; 2 1/2 teaspoons kosher salt, divided; 1 large leek; 1/2 cup walnuts; 1 small bunch thyme; 4 slices thick-cut bacon (about 5 ounces); 2 tablespoons unsalted butter; 1 1/2 cups heavy cream; 1/2 teaspoon freshly ground black pepper; 1/2 teaspoon red pepper flakes; 2 ounces blue cheese , crumbled (about 1/2 cup)