Stir together first 4 ingredients until well blended. Spread 1 Tbsp. on each bread slice. Top with desired toppings. Cover with plastic wrap, and chill 15 minutes to 12 hours. Trim crusts from chilled slices, and cut each slice into 2 triangles.
Salmon-Cucumber: Cut 1 medium-size English cucumber into thin rounds. Cut rounds into half moons. Divide cucumbers and 4 oz. sliced smoked salmon among prepared bread slices. Gently press to adhere. Sprinkle with fresh dill to taste.
Vegetable Medley: 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.
Prosciutto-Asparagus: Cut 1 lb. blanched fresh asparagus into thin, ribbon-like strips, using a vegetable peeler. (See The Blanching Blueprint, below.) Divide asparagus and 4 oz. thinly sliced prosciutto among prepared bread slices. Gently press to adhere.
THE BLANCHING BLUEPRINT
1. BRING TO A BOIL. Wash and trim vegetables. Bring a large pot of water to a boil. Stir in kosher salt until the water tastes as salty as the sea (about 1 1/2 Tbsp. per every 5 qt. of water). The salt will enhance the flavor of the vegetables. While the water returns to a boil, fill a large bowl halfway with ice cubes. Add just enough cold water to make the ice float.
2. COOK AND SHOCK. Working with one variety at a time, boil vegetables until they are bright green and crisp-tender (1 to 2 minutes). Transfer to ice water to set the color and texture, using a slotted spoon or tongs. Chill until ice-cold. Cool tip: Try nestling a colander in the ice bath before adding the vegetables. That allows you to drain without bothering with pesky ice.
3. STORE AND SERVE. Drain vegetables, and pat dry with a kitchen towel. Blanched, shocked vegetables will retain their bright color and crisp-tender texture when chilled up to 4 days or frozen up to 3 months. Store in a zip-top plastic bag. Serve as crudités, use in stir-fries or pasta, or sauté.