In a small bowl, mix roughly 2 tablespoons of the olive oil with the honey, mustard, and balsamic vinegar. Season with salt and pepper to taste. Do not add the thyme or chili yet, as the solid herbs will clog the syringe.
Transfer about 1/4 of the marinade to a marinade syringe. Inject it carefully into the center of the pork fillet at a depth of 2–3 cm.
Add the thyme and chili to the remaining marinade and mix well. Place the pork fillet and marinade in a freezer bag, seal tightly, and massage to distribute the marinade throughout the meat. Refrigerate for 2 hours.
Cut the 2 onions into small pieces. Heat roughly 1 tablespoon of the olive oil in a large pan over medium-high heat. Sauté the onions for 5 minutes until softened. Transfer the onions and oil to a roasting pan.
In the same pan, sear the pork fillet on all sides over medium-high heat for 5–7 minutes until browned. Transfer the seared pork to the roasting pan with the onions.
Place the roasting pan without a lid in an oven preheated to 170 °C (338 °F). Roast for 15–20 minutes until a meat thermometer inserted in the thickest part reads 63 °C (145 °F).
Slice the pork into 2 cm thick pieces and serve garnished with the roasted onions and a fresh salad.
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