Step 1

Preheat oven to 350°F. Cook bacon in a large skillet over medium-high, turning occasionally to cook evenly, until crispy, about 8 minutes. Transfer bacon to a plate lined with paper towels to drain. Reserve 3 tablespoons drippings in skillet; discard remaining drippings. Add onions to reserved drippings, and cook over medium-high, stirring often, until onions are very tender and caramelized, 15 to 18 minutes.

Step 2

Stir together Swiss cheese, cream cheese, scallions, mayonnaise, thyme, cayenne, and caramelized onions in a medium bowl. Chop bacon, and add to onion mixture; stir gently to combine all ingredients.

Step 3

Spoon mixture into a lightly sprayed 13- x 9-inch baking dish. Bake in preheated oven, until hot and bubbly and cheese is melted, about 20 minutes. Remove from oven, sprinkle with fried onions; garnish with thyme. Serve hot with crackers and pretzels.