2/3 cup low-fat plain yogurt
3 tablespoons prepared horseradish
4 tablespoons extra-virgin olive oil
3 medium-large red onions, thinly sliced (about 2 pounds)
2 sprigs fresh thyme
Kosher salt and freshly ground black pepper
1 cup red wine
1/4 cup honey
1/4 cup red wine vinegar
1 garlic clove, crushed
2 tablespoons balsamic vinegar
4 large portobello mushrooms, (about 1 pound) stemmed
Lettuce leaves
4 whole grain buns, or whole wheat English muffins, split and toasted