Maple Syrup Rum Baked Beans

• 4 cored apples unpeeled
• 1 onion
• 2,800 ml water (cold)
• 1,100 gr navy beans (dried)
• 325 gr maple syrup
• 250 gr fat
• 220 gr maple
• 120 gr rum (optional)
• 110 gr butter
• 8 gr coarse salt