Peel the onions and keep the root end fully intact. Using a sharp paring knife, cut a shallow X about half an inch deep into the root end of each onion to allow the broth to seep into the center during cooking.

Melt the butter in a large Dutch oven over medium heat. Add the smashed garlic cloves and cook for about 1 minute until fragrant but not browned.

Pour in the broth and wine. Add the thyme sprigs, bay leaves, kosher salt, and pepper. Bring to a gentle simmer. Gently place the prepared onions into the pot root-side down. The liquid should reach about halfway up the sides of the onions. Add more broth or water if needed. Reduce heat to low, cover with a tight-fitting lid, and simmer gently for 45 to 55 minutes. Avoid lifting the lid frequently.

Test for doneness by inserting a paring knife or skewer into the center of the largest onion. It should slide in with no resistance. If any firmness remains, cover and continue cooking in 5-minute intervals.

Using a slotted spoon, carefully transfer the cooked onions to a serving platter and tent loosely with foil to keep warm. Raise the heat to medium-high and boil the cooking liquid uncovered for 8 to 12 minutes until reduced by about half and lightly syrupy. This step is optional but recommended.

Pour the reduced sauce over the warm onions. Garnish with fresh herbs and a pinch of flaky sea salt. Serve immediately with crusty bread. To serve as a main, place each onion in a shallow bowl with sauce over creamy polenta or mashed potatoes.