BBQ Pulled Chicken or Pork Sandwiches

Name: BBQ Pulled Chicken or Pork Sandwiches
Ingredients: Slow cooker ingredients:; 2 large bone-in, skin-on chicken breasts or a 3 lb pork shoulder roast; 1 teaspoon red pepper flakes; ½ tablespoon salt; 1 teaspoon freshly ground black pepper; ½ tablespoon yellow mustard seeds; ½ cup water; ½ cup vinegar; 2 yellow onions, thinly sliced in half-moons; 1 green bell pepper, seeded and finely chopped; For the barbecue sauce; ¼ cup unsalted butter; 1 large yellow onion, finely chopped; 2 cloves garlic, minced; 1 (15 oz) can whole tomatoes, pureed; 2 tablespoons ketchup; 1 teaspoon salt; ½ cup peach or apricot jam; ¼ cup dijon mustard; ¼ cup honey; ¼ cup dark brown sugar, firmly packed; ½ cup cider vinegar; 1 tablespoon worcestershire sauce; 1 teaspoon tabasco sauce (or more to taste); 1 teaspoon dried rosemary; Salt and pepper to taste; 6 sandwich buns, warmed; Pickle slices