1 cup mayonnaise
2 tablespoons sour cream
1 tablespoon freshly grated lemon zest
2 teaspoons coarse-grained mustard
2 teaspoons fresh lemon juice, or to taste
24 very thin slicess homemade-type white bread
1 cup mint leaves (from about 1 large bunch)
16 radishes, trimmed and sliced as thin as possible
In a small bowl stir together mayonnaisse, sour cream, zest, mustard, lemon juice, and salt and pepper to taste.
Spread bread generously with lemon mayonnaise. Top half of slices with a layer of mint and top mint with overlapping rows of radish. Top radish with remaining bread and press together gently. Trim crusts and cut sandwiches in half diagonally. Sandwiches may be made 2 hours ahead, wrapped in plastic wrap, and chilled.