In a large mixing bowl, combine the pork, red onion, rosemary, thyme, chilli powder (if using) and crumbled Stilton. Season with black pepper, then form into 4 burgers. Keep covered and chilled.
When ready to cook, preheat the grill, barbecue or char-grill pan. Cook the burgers for 8-10 minutes per side, depending on their thickness (they should be thoroughly cooked).
Split the burger buns and toast the cut sides, if you like. Top with salad leaves, then a burger. Finish each one off with a slice of Stilton and a drizzle of sweet chilli sauce, if you like.
For a great mini treat for parties and buffets, form the burgers into meatballs and bake in the oven at 200°C/fan oven 180°C/Gas Mark 6 for 20 minutes, then serve in warm pitta breads with salad and sweet chilli sauce.