Cut the onions into wide strips (French cut). Toss them with olive oil, balsamic vinegar, kosher salt and pepper.
Line a baking sheet with parchment paper. Place the onions on the baking sheet and roast 30 minutes. Remove from oven, stir and add the thyme.
Continue cooking 15 to 25 or so minutes, until tender. Taste and add another 1/8 teaspoon salt, plus more as necessary. Serve immediately or store leftovers refrigerated for up to 4 days.