Prepare these sandwiches up to 8 hours in advance, but don't add the avocado slices until ready to serve to prevent them from turning brown.
Stir together cream cheese and horseradish until well blended.
Spread cream cheese mixture evenly inside pita halves (about 2 tsp. each). Stuff pita halves evenly with lettuce, roast beef, onion slices, and avocado slices.
Find recipes with ingredients that you have on hand.