Combine first 4 ingredients in a small bowl, stirring with a whisk. Spread mayonnaise mixture evenly over 1 side of each slice of toast.
Place 1 slice on each of 4 plates, coated side up. Top each with equal amounts of turkey, onion, bell pepper, and sprouts. Sprinkle 1 1/2 teaspoons sunflower kernels over each sandwich. Top with remaining slices of toast, coated side down.