Preheat your oven to 400°F (200°C) and spread the pecans on a baking sheet. Toast them in the oven for 8-10 minutes, or until they are lightly golden and fragrant.

In a large mixing bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, stone-ground mustard, lemon juice, fresh dill, kosher salt, and black pepper. Mix until smooth.

Fold in cooked chicken, red grapes, Granny Smith apple, and red onion. Gently stir until evenly coated with the creamy dressing.

Slice the croissants in half horizontally. Fill each croissant with the chicken salad mixture, ensuring plenty of grapes, apples, and pecans are visible.

Serve immediately, arranging the sandwiches on a platter next to a light salad or fresh fruit for an elegant presentation.