2 lbs sweet onions (such as Vidalia, Walla Walla, Maui, or Texas Sweet)
Olive oil for grilling
1 Cup Balsamic Vinegar
1 Tbsp thyme leaf (preferably fresh)
1/2 tsp Pink Peppercorns, coarsely crushed
1/2 tsp Kosher salt (or more to taste)
Peel and trim the onions, and cut into 1/2-inch thick slices. Insert a bamboo skewer or wooden toothpick halfway into the onion round to keep the slices intact.
Prepare a grill for cooking on medium-low heat. As it heats, brush the grill grate clean and prepare it with a paper towel dipped in oil. Using a basting brush, brush both sides of each onion slice with olive oil and grill, covered, flipping every 10 minutes, until the slices are very soft and well browned on the outside, 35 to 40 min all together. (If the onions begin blackening, the grill is too hot; you’re going for a golden, brown, almost translucent look.) When browned, transfer the grilled onions to a foil-lined plate, and cover loosely with the tinfoil. Set aside while you make the glaze.
Meanwhile, pour the balsamic vinegar into a small saucepan along with the thyme leaves, pink peppercorns, and salt. Bring to a boil over medium-high heat and simmer, uncovered, until the vinegar mixture has reduced to about 1/4 cup and has a thick and syrupy texture, about 8 to 10 min. Let the balsamic mixture cool briefly before seasoning to taste with additional salt, if needed.
Transfer the onions to a serving dish. Gently remove the toothpicks along with any extra-charred layers, if necessary. Pour the warm balsamic glaze over the onions, using the basting brush to distribute it evenly across the surface of the grilled onions.
Can be served warm, or at room temperature. Makes a lovely accompaniment to grilled meats, poultry and fish, nice taste contrast to strongly-flavored salad greens and tomatoes, or atop bruschetta.
Make Ahead: The onions can be grilled, and the balsamic glaze cooked up to several hours ahead of serving. Wait to glaze the onions until just before serving. If glaze is too cool it will thicken — just reheat gently on the stove before drizzling over the grilled onions.