Generously salt the chicken legs and let rest for 30 minutes. Remove woody stems from shiitakes. Place chicken, mushrooms, and garlic in a deep pan and submerge in oil. Heat to 90°C (194°F) and cook on low for 1 hour until tender.
Sprinkle the gelatin powder over 2 tablespoons of cold water. Let it sit for 5 minutes until it becomes a thick paste.
Remove chicken from oil, discard skin and bones, and shred the meat. In a large bowl, combine the shredded chicken, confit mushrooms, two smashed confit garlic cloves, chopped walnuts, and thyme. Season with salt and pepper.
Heat the chicken stock until steaming. Whisk in the bloomed gelatin until dissolved. Pour the stock over the chicken mixture and stir. Line a mold with cling film, press the mixture in firmly to remove air, and weigh it down. Refrigerate overnight.
Sear onion rounds in butter and thyme until caramelized. Toss radish slices with lemon juice and salt. Keep apple slices in lemon water to prevent browning.
Cut a portion of the terrine. Place a warm onion round on the plate, top with an apple slice, and then the terrine. Garnish with radishes, a confit garlic clove, and a dot of whole-grain mustard.