Stir together cream cheese, lemon juice, grated onion, and 1/4 teaspoon salt until well blended.
Spread 1 Tbsp. of the cream cheese mixture on each bread slice.
Cut 1 medium-size yellow squash, 1 medium cucumber, and 10 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.
Cover with plastic wrap, and chill 15 minutes to 12 hours.
Trim crusts from chilled slices, and cut each slice into 2 triangles.
Serve with other varieties such as Prosciutto-Asparagus Tea Sandwiches to create a mosaic look.