Prepare the pork: Pat the pork tenderloin dry with paper towel and season generously with salt, black pepper, and fresh thyme leaves. Cut the tenderloin into 6 equal portions, each about 3-4 cm thick.
Sear the pork: Heat 2 tablespoons olive oil in a large pan over medium-high heat. Sear each piece of pork tenderloin for 2-3 minutes on each side until golden brown but not cooked through. Remove from the pan and set aside to rest and cool completely. This step can be done while preparing the onions.
Make the brandy caramelised onions: In the same pan, add 1 tablespoon olive oil and the sliced red onions. Sauté over medium heat for 8-10 minutes until the onions are soft and translucent. Add the brandy carefully (it may flame briefly) and let it cook off for 1-2 minutes. Stir in the cherry preserve, salt, and black pepper. Continue cooking for another 5-7 minutes until the mixture is thick, jammy, and caramelised. Remove from heat and allow to cool completely.
Prepare the pastry: Preheat your oven to 200 degrees Celsius. Line a baking tray with baking paper. Roll out the puff pastry sheets and cut each sheet into 3 large squares (you’ll need 6 squares total for 6 wellingtons).
Assemble the wellingtons: On each pastry square, lay 2-3 rashers of streaky bacon in the centre, slightly overlapping. Spoon 1-2 tablespoons of the caramelised onion mixture over the bacon. Place one piece of seared pork tenderloin on top of the onions. Fold the bacon over the pork, then bring up the corners of the pastry to meet in the centre, creating a parcel. Press the edges together to seal, ensuring no filling escapes. Place seam-side down on the prepared baking tray.
Decorate and glaze: If desired, cut decorative shapes (such as leaves) from leftover puff pastry and place on top of each wellington. Brush the entire surface generously with beaten egg for a golden finish.
Bake the wellingtons: Bake in the preheated oven for 20-25 minutes until the pastry is golden brown, puffed, and crispy. The internal temperature of the pork should reach 63 degrees Celsius.
Rest and serve: Remove from the oven and let rest for 5 minutes before serving. Garnish with fresh thyme sprigs.