Step 1

Place stock, onion, carrot, celery, bell pepper, butter, salt, black pepper, and crushed red pepper in a 6-quart slow cooker. Cover and cook on HIGH until vegetables are tender, about 3 hours.

Step 2

Whisk Cheddar cheese, cream cheese, and 1 cup of the milk into mixture in slow cooker. Cover and cook on HIGH until cheese is melted and smooth, whisking occasionally, about 30 minutes.

Step 3

Whisk together cornstarch and remaining ¼ cup milk in a small bowl until smooth. Whisk into soup in slow cooker. Cover and cook on HIGH, whisking occasionally, until thickened, about 15 minutes.

Step 4

Slice ½ inch from the top of each pretzel bun using a serrated knife. Scoop out centers of bun bottoms, leaving a ¼-inch-thick shell. Ladle soup evenly into bun bottoms; garnish with bacon and scallions. Serve alongside bun tops.