Crush the 2 garlic cloves. Rub the pork roast with crushed garlic and season generously with salt and pepper. Heat the 3 tablespoons oil in a large roasting pan over medium-high heat and brown the meat on all sides, about 8 minutes total.
Add the roughly chopped soup greens and chopped onion to the pan. Cook over medium-high heat, stirring occasionally, for 4 minutes until the vegetables start to brown. Add the caraway seeds and stir for 30 seconds.
Pour in roughly half of the broth. Preheat the oven to 200°C (400°F). Transfer the roasting pan to the oven and braise for 90 minutes, pouring in the remaining broth gradually throughout cooking so the liquid reduces to approximately 0.5 liters. Check that the pork has reached an internal temperature of 63°C (145°F).
Remove from the oven and let the meat cool to room temperature. Strain the sauce through a fine sieve into a clean pot. Season to taste with salt and pepper. If needed, add a little more broth to bring the total to 0.5 liters. Stir in the vinegar. Do not use thickeners, flour, or cream.
Add the aspic powder to the hot sauce. Let it soften for 1 minute, then stir until completely dissolved.
Once the pork is completely cool, cut it into 5 mm thick slices, then cut each slice into bite-sized pieces. Layer the meat pieces into three clean, sterilized 440 ml jars, dividing equally. Pour the hot sauce over the meat to fill each jar, then seal tightly.
Place the sealed jars upright side by side in a large pot and fill with water to come 2 cm below the jar lids. Cover the pot and bring to a boil, then reduce heat to maintain a gentle simmer at 98°C (208°F) and process for 90 minutes. Once properly sealed, the preserve will keep for several months.
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