Peel and dice the ripe mango into small 1/4- to 1/2-inch cubes. Add mango to the bowl with the chicken. Core and finely dice the red bell pepper and add to the bowl. Mince the red onion finely (so it doesn't overpower) and add along with the chopped cilantro and sliced green onion.

Make the Curry Dressing

In a medium bowl, whisk together 1/2 cup mayonnaise, 1/4 cup plain yogurt, 1 tablespoon honey, 2 teaspoons fresh lime juice, 1 tablespoon curry powder, 1/2 teaspoon ground cumin, and 1 teaspoon Dijon mustard until smooth and uniform.

Season the dressing with 3/4 teaspoon salt, 1/2 teaspoon freshly ground black pepper and 1/4 teaspoon red pepper flakes if using. Taste and adjust: add an extra 1/2 teaspoon lime juice for brightness or 1/2 teaspoon honey for sweetness if desired.

Combine Salad

Pour the dressing over the chicken, mango and vegetables in the large bowl. Gently fold until all ingredients are evenly coated, taking care not to break up the mango too much.

Stir in the 1/4 cup toasted slivered almonds (or cashews) and 1/2 cup diced celery. Taste and adjust seasoning: add more salt, pepper, or a squeeze of lime if needed. Chill the salad in the refrigerator for at least 15 minutes to let flavors meld (optional but recommended).

Toast Bread & Assemble Sandwiches

If toasting bread: lightly butter one side of each slice or brush with olive oil. Heat a skillet over medium heat and toast bread butter-side-down until golden, or use a toaster. Repeat for remaining slices.

Lay out 4 slices of bread. Place a lettuce leaf on each, then divide the mango curry chicken salad into four equal portions (about 3/4 cup each) and mound onto the lettuce. Top with another slice of bread to make a sandwich, or serve open-faced with additional lettuce on the side.

Optionally garnish each sandwich with extra cilantro leaves or a few extra toasted nuts for presentation. Serve immediately. Leftovers can be kept in an airtight container in the refrigerator for up to 2 days.