Add the whole chicken into a large bowl. Sprinkle the chicken with ¾ tsp salt, ¼ cup mustard, 2 tsp cumin, and ½ tsp black pepper. Stir around with your hands to fully coat the chicken, then set aside momentarily.
Add 3 tbsp vegetable oil to the bottom of a dutch oven pan and allow it to heat up. Add the chicken into the pot and fry in the oil for about 3-5 minutes on each side, until they are browned on each side.
To make the sauce, blend 5 roma tomatoes, ½ a yellow onion, 2 guajillo chilies, 3 garlic cloves, 3 cloves, 1 tsp achiote, 2 tbsp sesame seeds, ⅓ cup almonds, 1 tsp cumin, ½ tsp oregano, 1 tsp salt and 1 cup water. Blend until the sauce is completely smooth.
When the chicken has fried, add the sauce you made in the blender and 2 bay leaves. Add a lid and put the pot over medium heat. Let it cook for an hour.
After an hour, turn off the heat. Use tongs to remove the chicken meat from the bones. Discard the bones but keep the chicken in the saue.
Once the bones have been removed, leave the chicken covered on the stove (turned off) while you prepare the sauce.
Add all of the dressing ingredients into a bowl and use a whisk to combine.
Prepare the sandwiches by cutting a slit in the top of the bun and opening the bun from the middle. Spread the dressing around inside the bread.
Use a piece of lettuce to line the inside of the bun. Then add the chicken & sauce into the lettuce. Place the veggies (tomatoes, cucumbers, radishes, watercress leaves, etc) inside the bun.
Repeat until the sandwiches are assembled and enjoy!