Place onions in a large heatproof non-metallic bowl, cover with boiling water; stand for 30 minutes, drain. Peel onions, leaving root ends intact; return to bowl.
Combine salt and the boiling water in a large heatproof jug; stir until salt dissolves. Add the cold water; pour over onions. Cover with a large plate or a sealed plastic bag half-filled with water to keep onions submerged; stand overnight.
Place garlic, peppercorns, vinegars, sugar and 2 sprigs of both rosemary and thyme in a 4.5-litre (18-cup) slow cooker. Cook, covered, on high, for 2 hours.
Meanwhile, rinse and drain onions; pat onions dry with paper towel. Pack onions, bay leaves and remaining rosemary and thyme into hot sterilised jars.
Discard rosemary, thyme and garlic from vinegar mixture. Pour enough vinegar mixture into jars to cover onions; seal immediately. Label and date jars when cold.