1. Stir together first 4 ingredients until well blended. Spread 1 Tbsp. on each bread slice.
2. Cut squash, cucumber, and radishes into thin rounds. Cut rounds into half moons. Divide vegetables among prepared bread slices. Gently press to adhere. Sprinkle with cracked black pepper to taste.
3. Cover with plastic wrap, and chill 15 minutes to 12 hours. Trim crusts from chilled slices, and cut each slice into 2 triangles.