Preheat the oven to 375 degrees F. Whisk the mayonnaise, ketchup and pickle in a large bowl until combined; season with salt and pepper.
Add the Swiss cheese, sauerkraut, cream cheese and pastrami to the mayonnaise mixture; stir with a rubber spatula until combined. Using a small serrated knife, hollow out the bread loaf, leaving a ½-inch-thick shell; reserve the cut-out bread. Fill the loaf with the dip; transfer to a baking sheet.
Slice the reserved bread into pieces and arrange on a separate baking sheet; drizzle with olive oil and season with salt. Bake the bread bowl until the cheese melts and the top is golden brown, about 30 minutes, adding the sliced bread to the oven during the last 10 minutes. Serve with the toasted bread and crackers and/or crudites.
Photograph by Levi Brown