Heat oil in large skillet over medium-high heat. Add onions and sauté, stirring frequently, until lightly caramelized, about 20 minutes. Reduce to medium-low; continue to cook until impressively caramelized, about 10 minutes longer. Stir in thyme, a generous sprinkling of salt and pepper, and the apple butter. Cook to heat through.
Meanwhile, adjust oven rack to lower-middle position and heat oven to 350 degrees F. Top each bun bottom with a portion of cheese and spread each bun top with a portion of mustard. Top cheese with a portion of onions and cap each bun. Bake until cheese melts, 8 to 10 minutes. Serve.
Recipe from Pam Anderson.
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