Roasted Root Vegetable Potpie

Name: Roasted Root Vegetable Potpie
Ingredients: 1 large yellow onion, quartered and sliced crosswise into ½- to ¾-inch-thick pieces; 1 tablespoon kosher salt, divided, plus extra if needed; 2 cups vegetable or chicken broth, divided; 12 large garlic cloves, peeled, plus 2 garlic cloves, very finely chopped; 6 medium carrots, peeled and sliced on a bias into ½-inch pieces; 1 medium rutabaga, peeled and chopped into ½-inch cubes (about 4 cups); 1¼ teaspoons freshly ground black pepper, divided; ¼ cup extra-virgin olive oil, divided; 5 fresh thyme sprigs plus 2 teaspoons finely chopped fresh thyme, divided; 2 tablespoons unsalted butter, at room temperature; 3 tablespoons all-purpose flour; ½ medium head napa cabbage, sliced crosswise into ½-inch-thick pieces (about 6 cups); 8 medium radishes, trimmed and quartered; 1 large egg; 1 sheet frozen puff pastry, thawed; Flaky salt