Preheat oven to 425°F. Grease a mini muffin tray with spray; set aside.

In a bowl, stir together finely grated potato, egg, parsley, salt, and pepper. Using very small amounts of the grated potato mixture (squeeze out any excess moisture), press into the bottom of each prepared muffin hole, pressing up sides somewhat; repeat with remaining potato mixture, making about 18 nests. Drizzle half a teaspoon of oil over every nest.

Bake in center of preheated oven for 20 minutes or until golden and crispy. Cool for five minutes. Using a small paring knife, carefully remove nests from pan. Let cool on paper towels.

Meanwhile, in a bowl, stir together roast beef, cucumber, green onion, coriander, and lime juice. (Meat mixture can be covered with plastic wrap and refrigerated for up to eight hours. Bring to room temperature before continuing).

Spoon a small amount of meat mixture into nests. Top with a quarter teaspoon of horseradish. Serve immediately.