Heat butter in large skillet over medium-low heat. Add onions and thyme sprigs; cook 30 to 45 minutes or until dark golden brown, stirring frequently. Remove from heat. Remove and discard thyme sprigs.
Place cheeses and cornstarch in large zip-top bag and shake to combine.
In medium saucepan, heat garlic and wine to a bare simmer over medium-low heat. Add the cheese mixture, a small handful at a time, stirring constantly, until all cheese is incorporated and mixture is smooth. Stir in caramelized onions. Season with salt and pepper, and serve immediately with pretzel bites.
Notes
Onions can be caramelized up to 3 days in advance. After cooking, let stand at room temperature until completely cool. Transfer to covered container and refrigerate. To finish recipe, reheat onions in microwave or small skillet until warm, then proceed with Step 2.
This website provides estimated nutrition information as a courtesy only. You should calculate the nutritional information with the actual ingredients used in your recipe using your preferred nutrition calculator.