Season pork belly with salt, pepper, and cayenne. Place fat-side down on a piece of parchment paper; fold paper around pork, then wrap in a few layers of foil. Place in a baking dish, fat-side up.
Bake in the preheated oven until internal temperature reaches 165 degrees F (74 degrees C), about 7 1/2 hours. Let cool completely, still wrapped, then transfer to the refrigerator.
Drain the soaked black-eyed peas and place in a pot with 2 quarts water. Bring to a simmer over high heat. Add bay leaf and salt. Reduce heat to medium-low and cook until beans are just tender, 35 to 40 minutes. Drain beans and set aside.
Whisk Dijon mustard, white vinegar, rice vinegar, and oil together in a bowl. Add garlic, onion, jalapeno, and bell pepper. Pour warm beans into the dressing. Season with salt, pepper, and cayenne; toss to combine. Wrap salad in plastic wrap and chill to let flavors blend, 8 hours to overnight.
Mix parsley into the salad and taste for seasoning.
Unwrap pork belly; cut into bite-sized chunks. Transfer pieces into a dry pan over medium heat. Cook, stirring occasionally, until lightly browned and crispy, 5 to 10 minutes.
Place salad in serving bowls and top with pork belly.
